Saturday, December 31, 2011

The Deltoid Area: Soft Shoulder and Varied Terrain

It's often assumed that the tissue landscape of the human shoulder is entirely muscle. When we draw a muscular individual, we cover his or her shoulders with lumps and bumps that represent lots of rippling beef. It's easy to forget that the terrain in this area is actually more varied; if you look closely (and palpate, if the muscular individual doesn't mind!) you'll observe a wide flat area where bone comes right up to the surface. What we're seeing (and feeling?) there is the acromion process of the scapula, a flat horizontal process that, while serving as both origin and insertion points for muscles, is not obscured by those muscles. This bony landmark is even more apparent when the arm is abducted; the deltoid muscle is primarily responsible for arm abduction, and as it bulges out during this action, its contracted form around the flat acromion process makes the latter stand out even more clearly.

Let's take a look at the appearance of the acromion process in the photo below. Note how it remains flat while muscle tissue bulges out around it.

This individual's deltoid and trapezius muscles are contracted because he is raising his arm over his head. In between these muscles, we see the flat acromion process, which serves as a partial insertion point for trapezius and a partial origin point for deltoid. When the these two muscles are contracted, the acromion process of the scapula (labeled A.P.) becomes quite pronounced.

Now lets look at a simple diagram in which the scapula landmarks and the basic shapes of the surrounding muscles are shown:

Bony and muscular landscape of the shoulder in a raised arm.
A.P = acromion process; E.C.R.L. = extensor carpi radialis longus.

This image displays two exposed bony portions of the scapula, the acromion process and the spine. It also displays the surrounding visible muscles, including those that attach to these scapula landmarks, the trapezius and the deltoid

Varied Terrain: The trapezius muscle is primarily a back muscle and most of it cannot be seen in this image. But its upper fibers, those that attach to the scapula, can be seen here. This portion of the trapezius inserts directly onto the spine of the scapula, the acromion process of the scapula, and also the lateral half of the clavicle. It does not, however, obscure any of these bony landmarks. The deltoid, coincidentally, originates at all these bony landmarks-- and it doesn't obscure them either. So these bony features remain visible just under the skin. As we move from the neck, over the shoulder, and down onto the arm, there is varied terrain: First we have the softly curved sweep of the trapezius along the neck, then we have the acromion process of the scapula forming a hard flat area on the shoulder, and then we have the long convex curve of the deltoid forming the rest of the shoulder.

Merge: As we see above, the deltoid's origin is very wide; it originates under the entire length of the scapula's spine, the acromion process, and the lateral half of the clavicle! Its insertion on the lateral humerus, however, is very narrow. So the deltoid's fibers converge together and it narrows to sort of a point before it inserts. The wide origin and narrow insertion of the deltoid form a triangular shape-- hence the name deltoid, which means delta-like in shape (as in the Greek letter delta.)

Three Lanes: You'll also observe in the diagram above that the deltoid muscle has three distinct sections, each named for its origin point. The anterior portion of the deltoid originates under the lateral half of the clavicle, the acromial portion originates at the acromion process of scapula, and the posterior portion originates under the spine of the scapula. Remember, none of these bone features are obscured by the muscle, which is why we can see them on the surface the body.

This Way: The pointed insertion end of the deltoid is a nice orientation landmark because it helps us find another muscle-- the brachialis. The brachialis is fairly easy to locate anyway, because it lies directly under the biceps brachii muscle, which is easy to locate on the anterior surface of the upper arm. The brachialis is shorter than biceps brachii, but a little wider, so it can be seen peeking out on either side. It's easier to spot on the lateral side because a) it shows more clearly there, and b) as mentioned above, the insertion end of the deltoid points right to it. 

No Turns: The brachialis muscle is, like biceps brachii, an arm flexor, but it does not supinate the arm like biceps brachii. Biceps brachii can supinate the arm as well as flex it because part of it inserts onto the radius, the bone responsible for forearm supination and pronation. Brachialis, however, inserts onto the coranoid process of the ulna and pulls it proximally, thus flexing the arm but not supinating.

The only other muscle that can be seen on the lateral arm is the triceps. Triceps is a three-headed muscle (tri = 3, and ceps = heads) meaning it has three distinct sections that come from three different origin points. While these origin points are obscured by other muscles, the three heads of the triceps muscle are still easy to distinguish. As expected, the lateral head is visible in this view of the lateral arm. One last note: The triceps muscle is the widest upper arm muscle, so some portion of it can always be seen, even from a straight on anterior view. In the lateral view above, we can see the triceps most posterior, the biceps brachii most anterior,  and the brachialis sandwiched in between them.

We'll look at the triceps muscle more closely in a future post, not to mention the back, including the entire trapezius muscle. This is my last post of 2011, and I'd like to wish you all a happy new year! If you're headed out tonight, please travel safely and obey those road signs! See you in 2012!

Friday, December 16, 2011

Anatomical Terminology 101: Baby Needs Some Direction

This blog is seven months old now and has slowly and imperceptibly transitioned from its infancy to its... um... toddlerhood? Not sure how to complete that metaphor except to say that my baby is beginning to stand on its own two feet and get around by itself. It's getting hits from all over the world without my having to hold its hand! Ah, now I'm all misty. (ahem) OK, in any case, now that we're standing on our own, maybe this is a good time to go over some basic anatomical terminology that describes overall  direction and location on this newly upright body. 

The first day of my Anatomy course at the American Academy of Art is typically spent going over direction and location terminology-- words that allow us to describes positions of anatomical structures and their relationships to one another without having to rely on pointing. This can be useful when visuals aren't an option. In addition, it's more accurate (not to mention more eloquent) to describe a structure as residing "on the distal end of the ulnar forearm" than "way down on the end of the arm, on the side by the pinky finger."

Let's start with a look at the body's midline, which is an imaginary line that runs down center of the body from either a front or back view; it's a line on either side of which we are basically symmetrical. While no one's body is perfectly symmetrical, most of us have the same basic form mirrored on either side of the midline: We have two ears, two eyes, two arms, etc. As such, there is no midline running down the side of the body, as the front of our body is different from the back.

The above image is an anterior view of the human body. The anterior side of the body is basically the front-- everything from the forehead down to the toes. The back of the body is known as it posterior side.  The posterior side includes everything from the back of the head down to the heels.

Our next two words, medial and lateral, are based on the concept of the midline. Medial and lateral are comparative terms that define a structure's location relative to the midline. A structure that is more medial is closer to the midline, and a structure that is more lateral is farther from the midline. So we might say that the corner of the mouth falls about two centimeters lateral to the midline. (Of course, every structure is lateral to the midline, because the midline is as medial as we can get.)

These terms occur more commonly in anatomical structures that come in pairs, such as the medial and lateral epicondyles of the humerus bone. The epicondyles of the humerus are bumps on its distal end, which is the end of the humerus near the elbow. (The terms distal and proximal will be explained later in this post!) The medial and lateral epicondyles are named as such because they are similar structures that need to be distinguished from one another. This happens a lot in human anatomy-- two or more structures will be very similar but not quite the same, so their names will be similar, too, but with one distinguishing qualifier. In the case of the epicondyles of the humerus, the qualifiers are medial and lateral. We have a medial epicondyle, which is the one closer to the midline, and a lateral epicondyle, which is the one one farther from it. 

These terms are used elsewhere in the body as well, such as the medial and lateral malleoli on the ankles (bumps on the tibia fibula) and the medial and lateral canthi (corners) of the eye. Now that you've read the last paragraph, you should be able to tell which is which.

You'll notice I also pointed out the terms anterior and posterior in the image caption above. The anterior side of the body is basically the front-- everything from the forehead down to the toes. The posterior side of the body is the back-- everything from the back of the head down to the heels. Both images in this post show anterior views of the human body.

Two more terms, superior and inferior, define relative position on the body in a different way. The superior end of the human body is the top of the head, and the inferior end is the bottoms of the feet. So a structure that is more superior is closer to the head, and a structure that is more inferior is closer to the feet. One example of the use of these terms is in the structure names superior vena cava and inferior vena cava. The vena cavae are the largest (and most cavernous) veins in the human circulatory system. They are named superior and inferior because one drains into the top of the heart (superior to it) and the other drains into the heart's underside (inferior to it.) We also use the roots supra- and infra- within anatomical terms to define relative structure positions. For example, the supraclavicular fossa is a depression in the skin just above the clavicle, and the infraorbital foramen is a hole in the maxilla just inferior to the orbit.

The last two direction and location terms we'll cover today are proximal and distal. These words describe relative location on a limb. We can't use superior and inferior for this, because that would change depending on the position of the limb. So we use a terms that don't depend on that: No matter what the position of the limb, there is always one end closer to the torso and one end farther from it. The proximal end of a limb is that closer to the torso. (The root prox- means near, and we see it in other terms such as approximately and proximity.) The distal end of a limb is farther from the torso. (The root dist- means farther or more distant.) So the proximal end of the arm is up by the shoulder, and the distal end of the arm is the fingertips. The leg's proximal and is up by the hip, and its distal end is at the toes.

We can also apply proximal and distal to individual structures in the limbs. For example, the femur has a proximal and and a distal end. And the tibia has a proximal end and a distal end. And we'd describe their relationship by stating that the distal and of the femur articulates with the proximal end of the tibia. 

Well, I've had enough of this, how about you? There are plenty more direction and location terms to cover, but let's save it for later. I'm thinking about getting a babysitter and skipping ahead to a shoulder post next... which would be the proximal end of the arm, right? See you next time.

Thursday, December 1, 2011

Landmark Sightings, Part 1: Bruce Lee

We just finished the arm portion of my fall anatomy classes, so I thought I'd get Bruce to help me with a little recap. First the image with some muscle overlays. Not a great deal of detail here-- just the basic shapes.

Behold Bruce, a fine source of landmark sightings. Click for a full view if necessary.

Now the image with labels but without the muscle overlay:

Worth noting: Muscle striations can be seen in the deltoid muscle. The extensor carpi radialis longus muscle bulges out more than any other on the forearm, so it's casting a deeper shadow than the rest. Extensor digiti minimi, a very thin muscle that extends the pinky finger, can be seen clearly between extensor digitorum and extensor carpi ulnaris. On the anterior upper arm, the cephalic vein can be seen popping out on the biceps brachii muscle, which it runs over just before entering the deltoid furrow, a crease between the deltoid and pectoralis major muscles. On the posterior upper arm, the division between the lateral and longs heads of the triceps muscle also shows clearly. This isn't usually the case, but Bruce is quite defined!

Image courtesy Thanks!

Monday, November 28, 2011

Anterior Leg, Part 2: It's Lonely at the Top

Now that we've covered the skeletal foundation of the lower leg, we're free to move on to its musculature with reckless abandon. First, some general information: Muscles in the lower leg (and in all limb sections) are grouped into compartments, each separated from one another by an enclosing layer of fascia. Within each compartment is a specific muscle group. Muscles compartments are typically named by location (anterior, posterior, medial, lateral, etc.) while muscle groups are named for their function (adductors, flexors, extensors, etc.) It makes sense that muscles within the same functional group fall within the same physical compartment because muscles would have to have similar origins and insertions (and thus similar locations) to have similar functions.

The lower leg has three muscle compartments-- the anterior, the posterior, and the lateral. In each of these fall muscle groups, each with its own functional purpose: In the anterior compartment we find the foot extensors and dorsiflexors; in the posterior compartment we find the foot plantarflexors, and in the lateral compartment we find the foot everters. A later post will elaborate on these movements. 

Today we'll be discussing the anterior compartment of the lower leg, but only its muscles that actually appear on the lower leg. Some muscles in the anterior compartment, while they lie in the lower leg, don't show up on its surface. Their tendons may show, but they don't surface until they've already reached the foot. Those particular muscle tendons are discussed in The Dorsal Foot: How Do I Love Thee? Let Me Count Your Tendons. The tibialis anterior, it turns out, is the only muscle whose body can clearly be seen on the anterior surface of the lower leg. It's on top of everything else, and it stands completely alone. 

DO YOU KNOW WHAT THIS MEANS??? It means that after this long, long, boring introduction, we're only going to cover one muscle today-- the tibialis anterior. As you may remember, I was planning to cover the tibialis anterior muscle last time but quickly realized it was impossible without first going over the lower leg bones; although tibialis anterior stands alone muscularly, its relationship with the tibia is the key to its identification.

So... let's start with an overview of the muscles in this area and their relationship to the bones covered last week in The Anterior Leg, Part 1: The Supporting Cast.

The entire medial surface of the tibis is exposed, but the lateral surface is obscured by the tibialis anterior muscle.

As we saw in the last post, the tibia is the larger and more medial of the two lower leg bones. There is a long ridge down its anterior side known as the anterior crest. One either side of the anterior crest are two long, flat surfaces. The medial of these (called, um, the medial surface) is completely exposed. It comes right up to the surface of the body, and it's what we colloquially refer to as the shin.

The lateral surface of the tibia is not a surface landmark because it is almost entirely obscured by, YES, the tibialis anterior muscle! This lovely little structure moves the whole foot, and is the only muscle in the anterior compartment to do so. (The other muscles in this compartment move the toes.) Because tibialis anterior is in the dorsiflexor group, it dorsiflexes the foot, or points it upward. This is not typically a very strong or pronounced foot movement, but it is important in making sure our foot is lifted up enough with each step so that we don't drag our toes. Yes, this little muscle keeps us from stubbing our toes. (Well, most of the time, anyway.)

Let's take a look at the tibia and the tibialis anterior's appearance on the lower leg:

The tibialis anterior muscle and its tendon can be seen very clearly on the surface of the leg when the foot is dorsiflexed and inverted. We can also see the vast surface of the medial tibia, as well as several other bony landmarks.

Notice how the lateral side of the lower leg appears soft and rounded, while the medial side appears flat and smooth. This is because the lateral side is soft tissue (in the form of the tibialis anterior muscle) and the medial side is the long, wide medial surface of the tibia.

Notice also how the tibialis anterior tendon shows clearly from just below the muscle body all the way down to the medial foot. It's most prominent just over the ankle. Notice also that the tendon of the extensor hallucis longus muscle runs parallel with that of tibialis anterior on the dorsal foot. We can tell one from the other, though, because the tendon of tibialis anterior is wider and more medial, and it surfaces more proximally than the tendon of extensor hallicus longus.

I have also pointed out a few other surface landmarks in the photo above, including certain features of the tibia and some dorsal foot tendons that come from anterior leg muscles whose bodies we cannot see up in the leg. 

In case the basic muscular and bony shapes need to be clarified, take a look at this very simple diagram, in which the basic bone and muscle shapes are overlaid onto the photo:

One last thing: Did you notice there is no medial compartment in the lower leg? Although we can invert our foot (turn its sole inward) there is no specific compartment whose function is only this. It makes sense that muscles on the medial side of the leg would invert the foot-- or pull it medially-- but alas, there is no medial compartment. But it turns out a medial compartment is not necessary here, because two other muscles on the lower leg take care of inversion. Gastrocnemius (in the posterior compartment) helps with inversion of the foot, and so does our friend tibialis anterior. 

This means tibialis anterior and its tendon really show when we are both dorsiflexing and inverting at the same time (or pointing the foot upward and inward at the same time.) Notice the foot in the photos is held in that position to ensure the best possible view for the camera.

So now we're familiar with our first lower leg muscle compartment. We'll move on to the posterior and lateral compartments in upcoming posts, but I think we might first take a short break from the leg and spend a little time going over the basic terminology of direction and location on the human body. This will help define a great deal of the words used over and over again in these posts. Until then, be sure to thank lonely little tibialis anterior next time you walk without stubbing your toe.

Sunday, November 20, 2011

Anterior Leg, Part 1: The Supporting Cast

Since we've covered some lateral knee and thigh structures and are due for more leg posts, I thought we'd dip down and visit the lower leg today-- specifically the anterior side. While this our the first visit to this area, it's not the first time we've mentioned the most prominent muscle here-- the tibialis anterior. This muscle is a star! Its body sweeps down the lateral surface of the tibia and stands out strikingly in foot dorsiflexion. Its tendon is even more visible on the antero-medial ankle as it courses down to the medial side of the foot. We've actually observed the tibialis anterior tendon before, in The Dorsal Foot: How Do I Love Thee? Let Me Count Your Tendons.

The tibialis anterior muscle was going get top billing in this post until it occurred to me that describing it was next to impossible without a thorough explanation of its supporting cast, the lower leg bones. So we'll examine those today and move on to a more detailed explanation of tibialis anterior next time.

The two bones of the lower leg are the tibia and the fibula. (Not fibia!) It's easy to distinguish these two bones from one another: The tibia is the wider of the two and lies more medial. It's the second longest and strongest bone in the human body (after the femur.) The fibula is the narrower of the two and lies on the lateral side. The tibia supports most of the weight placed on the lower leg, but the fibula breaks more often-- usually at its distal end-- because it's so thin. 

In the above diagram, the structures labeled in green are bony surface landmarks, which means they come right up under the surface of the skin and are often visible and/or palpable there. Notice that everything on the medial side has earned landmark status. Have you ever noticed that the medial side of your lower leg is much bonier than the lateral side? This is because the entire medial side of the tibia is completely exposed; nothing covers it other than skin, a little adipose tissue, and a thin layer of connective tissue.

The tibia, unlike other long bones, is not cylindrical in form. If cut transversely across its middle, its cross section would look more like a rounded triangle than a circle. The point of this triangle that faces anteriorly forms the anterior crest of the tibia, a long ridge down its anterior side. There are flat surfaces on either side of this crest, one lateral to it and one medial to it. The lateral surface of the tibia is not a surface landmark because it is almost entirely obscured by the tibialis anterior muscle. The medial surface of the tibia, however, is completely exposed. The tibia's medial surface and its anterior crest together form what we think of as the shin. And its complete exposure is what makes shin bumps so painful.

As we can see in the diagram above, the tibia's anterior crest and medial surface come right to the surface of the body, while its lateral surface is covered by the tibialis anterior muscle. Note also that the fibula is entirely embedded in muscle at this point (and along most of its length.) The only parts of the fibula that show on the surface of the human body are the head (at its proximal end) and the lateral malleolus (at its distal end.)

The above photo shows the appearance of these bony structures (among others) on the surface. Notice the entire medial tibia shows on the medial leg. (This will be more clear in the next post when we'll observe photos of the medial tibia juxtaposed against the tibialis anterior muscle.) We can also see the tibial tuberosity, a small bump just inferior to the patella, and the patellar ligament, which runs from the patella to the tibial tuberosity. We can also clearly see the medial malleolus of the tibia, which appears as a bump on the medial side of the ankle. Notice also that all we can see of the fibula on the lateral side are both ends of it-- the head proximally and the lateral malleolus distally.

Incidentally, this photo also shows lateral knee tendons (those of the iliotibial band and the biceps femoris muscle) which were discussed in The Lateral Knee: A Change of Scenery, and the lateral ankle tendons that were discussed in A Lateral Ankle Tendon: Peroneus Longus or Peroneus Brevis? Please visit these links for further information.

The tibia and the fibula provide the structural foundation for the muscular anatomy of the lower leg. Most of the lower leg muscle tissue is posterior to these bones, and we'll get to that soon. But next time, we'll take a good thorough look at the leading lady of the anterior leg, the lovely tibialis anterior! There might even be a sneak preview on the Human Anatomy for the Artist Facebook page. I'll get the popcorn and save you a seat down front.

Sunday, October 30, 2011

The Thoracic Cage: Halloween Skeletons Never Get It Right

Unfortunately my anterior leg photos turned out fuzzy, and I have no choice but to re-shoot them. While my exhausted camera recharged, however, I took the opportunity to finish putting up our Halloween decorations. This time of year is fun for everyone, isn't it? Kids get to trick-or-treat, parents get to see their young ones in adorable costumes, surly teens get to T.P. houses, and anatomy instructors get to giggle at all the bad skeleton decorations. That's right, giggle, my friends. You know, while I don't expect a $3.99 cardboard skeleton to be a paragon of anatomical accuracy, I do find myself wondering one thing every year: Why can't the Halloween skeleton artists ever give put in enough ribs?

Not only does this guy have only eleven ribs, but his elbow joints are clearly dislocated bilaterally. He might even have two radii his left forearm. In addition, each femur appears to be articulating with an obturator foramen instead of an acetabulum. Now that's spooky!

So, just to set things straight (for me, if for no one else) and as an extra special Halloween treat (yes, I do know how to party) let's review the thoracic cage and its surface landmarks!

The thoracic cage (a.k.a. rib cage) is part of the axial skeleton, whose purpose it is to provide protection of the vital organs. Each part of the axial skeleton has its own organ protection assignment; the skull protects the brain, the vertebrae protect the spinal cord (and also offer some abdominal organ protection posteriorly) and the thoracic cage protects the heart and lungs. Thoracic cage is a more accurate term than rib cage because this structure is more than just ribs; the thoracic cage is made up of ribs, the sternum, costal cartilage, and thoracic vertebrae.

The thoracic cage is wider laterally and flatter front-to-back. The uppermost ribs are very small and can't be seen on the surface of the body. As we descend to lower ribs, the thorax becomes wider; its widest point is right around the 8th rib. Then it narrows slightly again and the anterior side ends at the wide thoracic arch. While rib pairs three through six or seven come closer to the surface of the body, they don't typically show anteriorly because they're usually obscured by the pectoralis major muscle, which can be fairly thick. Ribs eight through ten are more likely to show on the anterior surface of the body, however, because they're covered by much thinner muscles, including external oblique.

It's a common misconception that the expansion and contraction of the lungs is what moves the ribs. On the contrary, it's the expansion and contraction of the ribs that fill and empty the lungs! The ribs move as a unit to facilitate respiration. As they lift and spread, the lungs fill with air. As they lower and compress, the lungs release air. The ribs can move like this because they articulate with other bones anteriorly and posteriorly with slightly moveable joints.

The Ribs
There are twelve pairs of ribs in the human thorax, and they are numbered from the top down. Each pair of ribs articulates posteriorly with a thoracic vertrebra (which are given that name because of their role as part of of the thorax.) This is why we have twelve thoracic vertebrae and twelve pairs of ribs. The joints at which the ribs articulate with vertebrae are called costovertebral joints. (costa is Latin for rib.) On the anterior side, however, only the first seven ribs articulate directly with the sternum, at joints known as costosternal joints. The seven ribs that articulate directly with the sternum are known as true ribs. The rest are known as false ribs. Ribs 11 and 12 are also known as floating ribs, because they don't articulate with any structure anteriorly.

The Sternum
Sometimes referred to as the breastbone, the sternum runs down the anterior midline of the thorax. It is made up of three separate pieces fused together at immovable joints. These pieces are named for their similarity to a sword. The most superior portion of the sternum is called the manubrium. This word is Latin for "handle," as this was evidently visualized as the handle of this sword shaped structure. The middle piece of the sternum, the body, is the largest portion of the sternum. Finally, the xiphoid process is the small bone at the inferior end of the sternum. Xiphoid comes from the Greek xiphoeides, which means swordlike. The xiphoid process may be either bony or cartilaginous in the adult human, and it often ossifies later than the rest of the bony skeleton.

Costal Cartilage
Shown in blue in the illustration above, the costal cartilage makes up the medial portion of the ribs on their anterior side. The costal cartilage makes this area of the thoracic cage more flexible. The thoracic arch, a surface landmark of the rib cage, is made up entirely of costal cartilage.

Thoracic Vertebrae
Running down the posterior midline of the thorax, the twelve thoracic vertebrae are considered both part of the spinal column and part of the thoracic cage. They are the only vertebrae with costal facets (flat articulation points for ribs) which makes sense, since no other types of vertebrae articulate with ribs.

Surface Landmarks
Several areas of the thoracic came form landmarks on the body's surface. How clearly they show depends on the amount of overlying tissue (either muscle or adipose) and the position of the body. Of all the thoracic surface landmarks, the jugular notch (a.k.a. suprasternal notch) is probably the easiest to see.

This figure study by American Academy of Art graduate Jacob Sanders shows a fine example of proper placement of the jugular notch. It is centrally located on the anterior neck, and often, on either side of it, we can see the knobby medial ends of the clavicles and the manubrial attachments of the sternocleidomastoid muscle. For more detailed information about this area, see The Anterior Neck: Theme and Variations.

Another typically visible thoracic surface landmark is the thoracic arch, a peaked arch of costal cartilage at the lower edge of the anterior thorax. This arch defines the superior border of the abdomen; it's the ridge where the bony thorax ends and the soft tissue of the abdomen begins. Its degree visibility depends on the amount of adipose tissue covering it, the thickkness of the muscles, and the position of the body. It will show more, of course, of the abdominal muscles are pulled in or if the rib cage is expanded due to inhalation. The thoracic arch also shows more clearly if the arms are held over the head or if the figure is lying supine.

Here is one of Jacob's illustrations in which we can see the thoracic arch:

One of the reasons Jacob figure work is so nice is that he, like Adam Nowak in a previous post, pays such close attention to anatomical detail. Here is a close up of the figure with the thoracic arch identified:

The position of the arms above the head is what make the arch more visible. We can also see some of the ribs in this image. Note that it's the first three false ribs (ribs 8 through 10) that show most. They're covered by the external oblique, a much thinner muscle than pectoralis major above, which usually obscures the true ribs.

Sometimes the ribs are also visible from a posterior view, as the back muscles covering them (trapezius and latissimus dorsi, for the most part) are relatively thin. Another of Jacob's illustrations demonstrates this. No need to even point out the ribs here. They're very clear:

Both of the flyers shown above are for Jacob's brother's band, Casket Showroom. Check them out! And again, to see more of Jacob's work, you can view his web site or his blog. Thanks for letting me use your work Jacob!

One last thoracic surface landmark is the sternal angle of Louis, which is a ridge at the level of the second rib, where the manubrium and body of the sternum meet. This landmark is usually only seen in very thin individuals--with low cut dresses! So maybe I'll cover this landmark the next time the Emmy Awards or the Oscars are aired.

Happy Halloween, everyone. Another leg post is on deck.

Friday, October 14, 2011

The Lateral Knee: A Change of Scenery

Hello! It's a lovely fall day here in Chicago, and another refreshing midwest change of scenery is upon us. While I love all the city offers during the warm summer months, the change of seasons is always welcome; too much of the same thing can get a little stale. This has me thinking that I could use a break from writing about upper extremity (as I'm sure you could use a break from reading about it.) As beautiful as the arm is, and as much as there is to learn about its structure, I think this week might be the perfect time for an anatomical change of scenery. Grab yourself a hot mug of apple cider and let's talk about the leg!

Recent news photos from a perennial fall event, the Chicago Marathon, got me thinking about an area of the leg I've been wanting to write about. On the lateral side of the knee, we can see two incredibly beautiful tendons whose surface appearance increases in clarity when weight is placed on the leg. So it's easy to see these tendons, as well as some surrounding muscles, on runners.

Let's start with a photo showing a lateral view of a runner's knee. Once you've recovered from the shock of this gentleman's extremely short shorts, you'll notice that two tendons show very clearly where the thigh reaches the knee. What we're seeing here are the insertions of the iliotibial band and the biceps femoris tendon.

While the thigh is heavy with strong muscles that completely obscure most of the femur, its lateral-most surface is covered with a wide tendinous sheath known as the iliotibial band. Just deep and posterior to that, we find the biceps femoris muscle, one of the flexor muscles on the posterior surface of the thigh.

The pronounced landmark tendons in the photo above stem off these two structures. The biceps femoris tendon is an insertion tendon that comes from, of course, the more proximal biceps femoris muscle. This tendon is posterior to the iliotibial band tendon, and it inserts onto the head of the fibula, just distal to the knee joint. The iliotibial band tendon comes from the iliotibial band above and it inserts onto the lateral side of the tibial head. These two tendons, when they protrude (most visibly on a weight-bearing leg) form a beautiful little fossa just proximal to the lateral knee. (In anatomical terminology, a fossa is a depression; the word fossa comes for the Latin for ditch.)

These two tendons are usually visible, but to varying degrees, as we'll see below. But first let's examine the anatomy more closely:

Let's first establish that this is a lateral view of the knee and lower leg. The fact that digit number 5 (the pinky toe) is closest to us makes this clear up front. But if we could not see the foot, we'd still know this was a lateral view because we can see both ends of the fibula (the head at the proximal end and the lateral malleolus at the distal end.) In addition, if we were viewing the medial side of the lower leg, we'd be able to see the entire length of the medial tibia, which is not obscured by any soft tissue. 

On the lateral knee we can see the two tendons that show in the runner photo above. The iliotibial band tendon comes from an eponymous band above. This band originates at the tensor fasciae latae muscle at the ilium (a pelvic bone), and it inserts onto the tibia, hence the name ilio-tibial band. We can also see that this band inserts onto the tibia just posterior to the patella.

The other visible tendon here is that of the biceps femoris muscle. It can be seen in this diagram just posterior to the iliotibial band. This tendons extends more distally than that of the iliotibial band because it inserts onto the head of the fibula. This feature of the fibula is a very nice orientation landmark because not only is it the insertion point for biceps femoris, but it's also the origin point for a lower leg muscle, peroneus longus. (Peroneus longus is briefly touched upon in a previous post, A Lateral Ankle Tendon: Peroneus Longus or Peroneus Brevis?)

We can see in the photo above, as well as the photo below, how a weight-bearing leg shows these tendons so clearly:

We can see here that the iliotibial band tendon is more anterior than the biceps femoris tendon, and it doesn't extend as far distally. Also, the iliotibial band tendon is wider and flatter than the more cylindricl biceps femoris tendon. Notice also how the biceps femoris tendon forms the lateral wall of the popliteal fossa, which is the hollow area on the back of the knee.

These tendons are still visible on a relaxed leg but in a different way. A painting below by my talented friend Adam Nowak shows this. First let's look at the full painting:

The model's right leg is relaxing over the left leg, and we can still see the lateral knee structures mentioned above. But here the iliotibial band reads as a sunken area because just anterior to it (or above, in this image) the relaxed vastus lateralis muscle is sort of bulging out over the iliotibial band, casting a shadow over it. Posterior to the iliotibial band (or below it, in this image) the biceps femoris muscle also bulges out as it's pressed against the right leg. The band itself, being of less flexible tissue, maintains its shape and reads as a flat crease.

Here is a close-up:

Notice the iliotibial band in the model's relaxed right leg reads as more of a long depression than a ridge, and the vastus lateralis muscle, although not contracted, bulges outward as its weight makes it sort of spill over the iliotibial band. The painter pays close attention to anatomical detail, and it shows here. You can see more of Adam's beautiful work at Adam Nowak's Art Blog.

I do miss summer a little bit, and I could spend another long stretch of warm weather hunched in front of my computer with a glass of iced tea, writing more about the arms. But a change of seasons is good, as is a change of scenery. There is much more to cover on the human leg, so let's stick around awhile and absorb the view. Another post will be up soon, possibly the anterior thigh or lower leg. Thanks to Adam for the use of his image! Until next time, my friends.

Saturday, September 24, 2011

The Dorsal Forearm: One Last Encore

I wasn't going to include these mini posts on the blog itself, but I kind of like this one, so here we go. We covered the dorsal forearm pretty thoroughly over the summer, but coming across this photo made me think maybe a quick encore was in order. Some of the muscles show pretty clearly here, so I slapped on some quick labels.

In the photo above, we can see the three most commonly visible dorsal forearm muscles, anconeus, extensor carpi ulnaris, and extensor digitorum. We can also see the lateral epicondyle of the humerus (the bony protuberance from which all these muscles originate) and the tendons of the extensor digitorum heading across the back of the hand to their insertion points on fingers II through V.

Compare the original photo to the labeled photo to get an idea of how clearly these structures can show, as well as where they appear and disappear. And don't forget that other variables (such as arm and hand position, age of the individual, and light source) will affect the surface appearance of these structures.

For more detailed description of this area, check out a previous post, The Dorsal Forearm, Part 2: Which Side Are You On, Anyway?

Some leg posts are coming up soon!

Wednesday, September 21, 2011

A Lateral Ankle Tendon: Peroneus Longus or Peroneus Brevis?

Hello! Just a quick post today to give you a taste of the extra anatomy information you can now get at the new Human Anatomy for the Artist Facebook page! Yep, I have a Facebook page now, on which I'll post links to all the full lessons that are normally seen on this blog, as well as other links, photos, book recommendations, and quick mini-lessons like the one below.

This will allow those who don't use Blogger (and those who use Blogger but don't check it often) to get updates on a more regular basis. The Blogger posts, after today, will resume their usual format of longer, more elaborate lessons.

So... today's mini-lesson is about a tendon seen on the lateral ankle and foot. Or is it two tendons? Let's take a look:

When drawing the lateral side of the foot, you'll almost always see a tendon up above (proximal to) the lateral malleolus of the fibula, which is a bony bump on the lateral side of the ankle. Sometimes, though, when the foot is everted (sole turned outward) and/or plantarflexed (toes pointed downward) you'll see what appears to be a continuation of that tendon down below (or distal to) the lateral malleolus. The whole thing really looks like one long tendon wrapping around the back of the malleolus. But... you guessed it. It's not!

What we're seeing here is actually two different tendons. The tendons of both the peroneus longus muscle and the peroneus brevis muscle wrap around the back of the lateral malleolus, but here's the weird thing. The peroneus longus tendon disappears right around the time it reaches the lateral malleolus. At that point, the peroneus brevis tendon emerges and continues its course along the lateral side of the foot. But the transition is so smooth that it looks like a single tendon both proximal to and distal to the lateral malleolus.

In drawing, the difference is that you'll almost always see the peroneus longus tendon, but the peroneus brevis tendon will usually only show when the foot is everted or plantarflexed.

One more thing: Some books call the peroneus longus and brevis tendons by a different name: fibularis longus and brevis. So if you see this, it isn't wrong. It's just an alternate name. Sometimes that happens in Anatomy. I guess it keeps things interesting.

We'll have a more detailed lateral leg post, complete with diagrams, soon!

Sunday, September 11, 2011

The Dorsal Forearm, Part 3: The Final Chapter

Hello, and welcome back! I am happy to say we will finally finish up the dorsal forearm today. Who knew such a small area of the body would require so many posts? We began with Dorsal Forearm: Compartment Search, in which we learned to identify the two compartments of the forearm-- an important first step in becoming oriented in such a complex muscular landscape. Then we learned about dorsal forearm muscles that are closer to the ulnar (pinky) side of the forearm in Dorsal Forearm: Which Side Are You On Anyway? As those muscles are easiest to identify, it was best we covered them first and then use them to find the remaining forearm muscles, which we'll cover today.

By the way, if you are interested in reading about the less complex tendinous landmarks of the ventral forearm, check out the very first post, The Ventral Forearm: What are those Tendons?

So. When we last left off in our forearm saga, we were looking at, among other structures, the "twin muscles," two muscles that look very much alike and run directly down the dorsal side of the forearm. Their similarity to one another as well as their central location on the dorsal forearm make them among the easiest to identify in this area.

The last three muscles we'll cover on the dorsal forearm can be found just radial to the twins-- meaning closer to the thumb side of the arm compared to the centrally located twins. It's no coincidence that each of these three muscles have the root "radial" in their names. As well as indicating that these muscles are found on the radial side of the arm, this root also tells us that these muscles pull the hand toward that side. This movement is known as abduction of the hand. Hence the fact that radial side arm muscles tend to abduct.

The three muscles we'll be looking for today are labeled in blue on this diagram. Notice that they are closest to the radial (thumb) side of the hand. And notice that all their names have "radial" somewhere in the name. 

Of these muscles, the most radial is brachioradialis. This muscle starts way up on the upper arm (or brachium, hence the root brachio in its name) and travels distally, along the radius, towards its insertion on the styloid process of the radius (a bump at its distal end.) This muscle, despite its certified membership in the dorsal forearm compartment, can actually be seen more clearly from the ventral side. So we won't see much of it today in our photographs.

Running right between brachioradialis and  extensor digitorum (one of the twin muscles) we see two muscles with very similar names: extensor carpi radialis longus and extensor carpi radialis brevis. Can you guess, by looking at these names, what these muscles have in common and what they don't?

Let's look at their names: Both names contain extensor carpi radialis, which means extensor of the wrist (carpi) on the radial side of the arm (radialis.) So these are attributes of both muscles. But how do we distinguish them from one another? The qualifiers tacked on to the end of each name tell us! Yes! One of these muscles is longer than the other. Extensor carpi radialis longus is the longer of the two, and extensor carpi radialis brevis is the shorter. (Brevis is Latin for short, or brief.) Knowing this, you should be able to tell one of these muscles from the other, as one is clearly longer than the other. In addition, it's helpful to know that extensor carpi radialis brevis lies right next to extensor digitorum and it tends to sink in rather than stand out when the dorsal forearm muscles show on the surface of the body.

Extensor carpi radialis longus is sometimes identified by its unique shape. It originates just proximal to the lateral epicondyle of the humerus, higher up on the arm than the origin points of the twin muscles. Also, unlike the twin muscles, extensor carpi radialis longus take a sharp turn where its muscle body meets its long insertion tendon. So its muscle body (which is its most visible part on the surface of the body) appears at an oblique angle on the upper dorsal-radial forearm. This is the only dorsal forearm muscle that lies at such an angle.

Key: Tlat: triceps, lateral head; Tten: triceps tendon; BrR: brachioradialis; ECRL: extensor carpi radialis longus; ECRB: extensor carpi radialis brevis; LE: lateral epicondyle; Anc: anconeus; OP: olecranon process; ED: extensor digitorum; EDM: extensor digiti minimi; ECU: extensor carpi ulnaris; FCU: flexor carpi ulnaris; APL: abductor pollucis longus; EPB: extensor pollucis brevis; EPL: extensor pollucis longus

The photo above shows a dorsal forearm and an abducted hand. Because radial side muscles abduct the hand, they will stand out more in this position than in any other. The easiest one to spot is extensor carpi radialis longus, as it stands out clearly right next to the lateral epicondyle. Notice its oblique course compared to the surrounding dorsal forearm muscles. Notice also that it and brachioradialis originate on the upper arm (unlike the twin muscles that originate at the lateral epicondyle of the humerus.)

At the distal end of the arm, you may also notice three smaller muscles, abductor pollucis longus, extensor pollucis brevis, and extensor pollucis longus. While these muscles stand out clearly here, they often don't. We will look at them more closely later. You may remember, however, that we have already observed their tendons (which can be seen at the base of the thumb) in the post on the dorsal hand.

I've also pointed out the lateral head of the triceps and the triceps tendon, structures on the posterior upper arm. We will cover these later, but this photo shows them very clearly.

Now let's take a look at this arm without the structure overlay:

Notice the most obvious radial side muscle is extensor carpi radialis longus. It bulges out more than its counterpart, extensor carpi radialis brevis. Brachioradialis does show fairly well on the surface, but as pointed out earlier, it actually shows more clearly on the ventral side of the forearm. Brachioradialis is one of the dorsal forearm muscles that can't decide which side it wants to be on; although it's technically in the dorsal compartment, it tends to peek around to the ventral compartment, and although it technically belongs to the extensor muscle group, it does facilitate some flexion of the wrist as well. In any case, we don't see it very clearly on the dorsal side of the arm, nor to we see extensor carpi radialis brevis very clearly (other than in an exceptionally defined individual.) As such, if the hand is abducted, the muscle we want to look for (and to be sure to draw!) is extensor carpi radialis longus.

So let's look for this muscle in a few more images...

In the photo above, we can see three structures very clearly. (Well, four, if you count the extensor digitorum tendons on the back of the hand.) We can see extensor digitorum (the more radial of the twin muscles), the lateral epicondyle of the humerus, and the extensor carpi radialis longus muscle. Notice again how ECRL runs more obliquely than its neighboring muscles and how it originates higher up on the arm than the twin muscles.

The extensor carpi radialis longus muscle is softer in this photo but still visible because the hand is abducted. We can also see the lateral epicondyle, anconeus, and both extensor digitorum and extensor carpi ulnaris. The degree to which the dorsal forearm structures are visible on the surface of the depends on several variables. These include but aren't necessarily limited to: the tone of the muscles, the amount adipose tissue overlying the muscles, the position of the arm and the degree of muscle contraction, the age of the individual (as it relates to skin thickness) and even the light source and the amount of contrast in the values of the structure.

Well, we've covered just about everything we can on the dorsal forearm, with the exception of the radial thumb muscles. Perhaps that can be our epilogue? But I'm going to put the forearm aside for awhile and move on to something different. I'm thinking maybe the posterior torso muscles or something with the thigh. We'll see. As always, suggestions are welcome!

Thanks to my forearm models, Christian, Jessica, and Jeff. I couldn't write this blog without your willingness to stand around and do funny poses for me.

Wednesday, August 17, 2011

How To Clean an Animal Skeleton

OK, I lied. I'm not posting the exciting conclusion of The Dorsal Forearm today. While I realize Part 2 left you with some massive cliffhangers (Are there really two extensor carpi radialis muscles?) you're just going to have to wait. Because I've once again been sidetracked while cleaning and organizing my home office. My last post on anatomical terminology was inspired by an old illustration; this post, my friends, is inspired by my growing collection of animal bones.

Over the years I've found quite a few partial animal skeletons in my garden, and several friends have been kind enough to give me any ossified treasures found on their property. I have mostly raccoons and possums, plus a few very delicate bird skulls. Having spent so much time learning about, drawing, and teaching classes about the human skeleton, I find it an interesting change of pace to observe these animal structures, in particular those of mammals, to look for features analogous to those of the human skeleton.

Before the bones are added to my collection, though, they need to be cleaned. Over the years I've come up with a pretty good method, and I thought I'd share it here, for any other anatomy lovers out there who want to hang on to a found skull or skeleton.

These are the skeletal remains of a possum I found recently. There is a partial skull, the ilia (which are
part of the pelvis) and two vertebrae. I mislabeled them, though. I think they are probably caudal
vertebrae, which are those in the tail.

So, here is the process I've been using for cleaning animal bones:

1) Have roommates who don't mind finding animal bones soaking in the sink.

2) Let the bones sit out in the sun long enough so any remaining particles of flesh are completely dried out. (It may already be at that stage when you find it!)

3) Gently break off as much of the flesh and fur that you can without damaging the bones.

4) Soak the bones in a bucket of soapy water overnight.

5) Use a soft toothbrush and a little soap to very gently scrub the bones. Then rinse them thoroughly.

6) Soak the bones overnight again, but this time in warm water with about 1/4 cup of bleach added to it.

7) Rinse the bones thoroughly.

8) Lay the bones on a towel and let them dry thoroughly. Letting them sit in the sun will speed up this process.

So there you go. Thanks to Dana for providing the skull above (I think?) Donated bones are welcome to my collection at any time! See you next time for another forearm post.

Monday, August 15, 2011

The Lovely Language of Anatomy and Medicine

The art students to whom I teach anatomy often cringe at the idea of learning structure names. "Why?" they implore, "Why do we need to know the names if all we're doing is drawing them?"

My goal is to show them that in order to discuss the structures that define the human body's shape, we have to have the language for it. We could simply point to different structures and call them "this one" and "this one" and this other one," but it wouldn't mean anything. It wouldn't stick.

And while it's true that my students will very likely forget many names of muscles and bones within a few months after completing my course (you know it's true!) I still believe that the fact that they once knew the names, that they once had to burn the impression of that name in their brain, will mean they'll never forget the structure itself. This may not be the most pleasant analogy, but it's sort of like learning the names of the structures is the actual wound, and remembering the structure and what it looks like is the scar. You need to go a little overboard initially in order to leave a lasting impression.

And aside from all that, I love the practicality of anatomical and medical terms. I mean, they're so descriptive! For example, take the muscle name extensor carpi radialis longus. It tells you just about everything you need to know! It's an extensor (extensor). It extends the wrist (carpi.) It's on the radial side of the forearm (radialis) which means it tends to pull the wrist in that direction as well. And finally, it is the longer of two similar muscles (longus.) By the way, its partner is known as extensor carpi radialis brevis; brevis means short, or brief.

There are many words in anatomical terminology that describe a muscle's function. Here are some examples:

- extensor (extends, or increases the angle of a joint) 
- flexor (flexes, or decreases the angle of a joint) 
- abductor (abducts, or pulls a structure away from the midline of the body) 
- adductor (adducts, or pulls a structure toward the midline of the body)
- tensor (tenses, as in tensor fasciae latae, a muscle that tenses the iliotibial band.)
- arrector (erects something, as in the arrector pili muscles, tiny muscles in the hair follicles which make hairs stand erect and give us goose bumps.)

There are also anatomical terms that describe different areas of the body, such as:

- cranial: having to do with the cranium (as in cranial nerves, cranial arteries, epicranius muscle)
- cervical: having to do with the neck (as in cervical vertebrae, those found in the neck)
- thoracic: having to do with the thorax (as in thoracic vertebrae, those attached to ribs)
- costal: having to do with ribs (as in costal cartilage or intercostal muscles)
- abdominal: having to do with the abdomen (as in the abdominal aorta or the rectus abdominis muscle)
- lumbar: having to do with the lower back (as in lumbar vertebrae, those found in the lower back)
- femoral: having to do with the thigh (as in the rectus femoris muscle, or the femoral artery)
- brachii: having to do with the upper arm (as in the biceps brachii muscles, or the brachialis muscle)
- carpi (wrist; as in extensor carpi ulnaris muscle, or the carpal bones)
- digitorum (fingers or toes; as in extensor digitorum longus muscle in the lower leg, which extends the toes)
- digiti minimi (small finger; as in extensor digiti minimi muscle in the forearm, which extends the little finger)
- pollicis (thumb; as in the many muscles that move the thumb: extensor pollicis longus and brevis, abductor pollicis longus and brevis, flexor pollicis longus and brevis)
- hallicus (big toe; as in extensor hallicus longus, the muscles that extends the big toe)

You'll notice some of the names of aforementioned structures included the term rectus, which brings me to anatomical terms which describe direction and shape:

- rectus: running straight up and down, as in rectus abdominis muscle and rectus femoris muscle.
- oblique: running at an oblique angle, as in external obliques and internal obliques.
- transversus: running transversely or side-to-side, as in the transversus abdominis muscle.
- deltoid: shaped like a Greek delta (triangular) 
- quadratus: four sided, as in the quadratus labii inferioris muscle (four-side muscles inferior to the lower lip) or the pronator quadratus muscle (a four sided muscle deep in the forearm that helps pronate it.)
- serratus: jagged or serrated in shape, such as the serratus anterior muscle.
- trapezius: Muscle on the back that is trapezoid in shape.
- rhomboids: As in rhomboid major and minor, muscles on the back that are shaped like rhombuses. Or is it rhombi?

Some more odd examples of structures named for shape are:

- gastrocnemius: belly shaped
- soleus: fish shaped (and some say this means sandal shaped.)

By now you'll have also noticed many muscle names that describe relativity between two similar muscles:

- longus, brevis (longer and shorter versions of similar muscles, such as extensor pollicis longus and extensor pollicis brevis.)
- maximus, medius, minimus (large medium and small versions of similar muscles, such as gluteus maximus, gluteus medius and gluteus minimus.)
- superficialis, profundus (superficial and deep versions of similar muscles, such as flexor digitorum superficialis and flexor digitorum profundus.)

There are also some pretty cool roots for different internal areas of the body, such as:

- gastro- stomach
- hepato- liver (as in hepatitis, which is inflammation of the liver, or hepatocyte, which is a liver cell.)
- arthro- joint (as in arthritis, which is inflammation of the joints)
- renal: related to the kidney (as in renal artery, or renal calculi, which are kidney stones)
- spondylo- spine (as in spondylitis, which is inflammation of the spine.)
- cardio- heart (as in cardiac muscle)
- pulmo- lungs (as in pulmonary arteries)
- cyto- cells (as in cytology-- the study of cells, or hepatocyte, which is a liver cell.)
- dermo- skin (as in dermatology, the study of skin, or epidermis, which is the outer layer of skin.)

Illustration of gallstones or cholecystolithiasis. Chole = bile; cysto = sac; litho = stones; iasis = process.  Cholecystolithiasis is the process by which stones are formed in the gallbladder, which is basically a sac of bile.

The last example of a beautiful and practical medical term brings us back to the image I found today. It illustrates gallstones, or cholecystolithiasis, whose name can be broken down like this: Chole- means bile; cysto- means sac; litho- means stone; iasis means process. Soooo... cholecystolithiasis is: The process by which stones collect in the gallbladder (which is essentially a sac of bile.) 

Ah, well I've gotten that out of my system. Until next time, anatomy lovers. I promise we will return to Part 3 of the forearm then!

Thursday, July 28, 2011

The Dorsal Forearm, Part 2: Which Side Are You On, Anyway?

Now that we’ve established a method for identifying the two compartments of the human forearm, let’s look at the dorsal compartment a little more closely. The dorsal side of the forearm is the "top" side-- the side that usually faces upward and is generally darker and hairier (due to more melanin and a greater number of hair follicles.) The muscles on the dorsal side of the arm (unlike those on the ventral side) are arranged in a single layer, so they all lie directly under the surface of the skin. This means most of them can be seen pretty clearly on a well defined individual, although their relative visibility will depend on the position of the hand. The muscles in this compartment are all part of the extensor/supinator muscle group, which means they either extend the wrist or fingers or supinate the forearm, or both.

When considering which muscles will show in a given position, we want to keep in mind what the muscles in a specific area tend to do. We are looking at the extensor/supinator group today, so it's safe to assume that these muscles can be seen more clearly when we are supinating the forearm (which means turning the palm upward) or extending the wrist or fingers (which means opening them up.) It's also helpful to remember that muscles on the radial (thumb) side of the arm tend to pull the hand in the direction of the radius (or abduct it) and muscles on the ulnar (pinky) side of the hand tend to pull the hand in the direction of the ulna (or adduct it.) As such, radial side muscles tend to show more when the hand is abducted and ulnar side muscles tend to show more when the hand is adducted. Knowing these four movements will help us to remember which hand positions will make which muscles stand out.

So... here is an overview of what we'll cover today:

When identifying muscle shapes on the forearm (or anywhere else on the body, for that matter) it’s best to locate the most obvious structures first and then find the other structures based on their relationships to the former. Today we'll look at the ulnar (pinky) side muscles, because I think they're easier to find first. Once we've established their location, we'll go on to the radial (thumb) side in the next post.

When becoming oriented on the dorsal forearm, I usually begin with what I like to call the twin muscles-- two muscles the seem to look more alike than any of the others. They are often the first to make themselves evident when looking at the dorsal forearm. The twin muscles are extensor digitorum and extensor carpi ulnaris. They both originate at the lateral epicondyle of the humerus (the bony bump on the lateral side of the elbow) they are about the same width, and they both run straight down the dorsal side of the forearm, toward the hand. Find them below in both the anatomical rendering and the photograph, and notice how similar they are.

The dorsal forearm "twin" muscles, extensor digitorum and extensor carpi ulnaris, look very much alike. We can distinguish one from the other by remembering that extensor carpi ulnaris is closer to the ulnar (pinky) side of the arm. 

Please note that the elbow position in the photo does not exactly match that in the diagram. But for our purposes, we can see what we need to see: The twin muscles and the furrow where the two muscle groups meet (shown with a dashed line.)

OK, so now that we’ve found the twin muscles, the next step is to distinguish them from one another. This is easy. The twin muscles are extensor digitorum and extensor carpi ulnaris. It makes sense that, of the two, extensor carpi ulnaris is closer to the ulnar side of arm. It actually lies right next to the ulna (whose location is designated by the furrow on the forearm, marked with a dashed line in the photo.) Once we've positively identified extensor carpi ulnaris, we can assume the other twin must be extensor digitorum.

The next muscle we'll locate is extensor digiti minimi. As its name tells us, this muscle extends the small finger (digiti minimi is Latin for smallest finger.) Extensor digiti minimi is very easy to locate because it's very narrow and lies right between the twin muscles. On the anatomical diagram, you can see that the tendon of this muscle is headed straight for the smallest digit, where it will insert on its dorsal side. This tendon is often visible on the dorsal hand. You can see an example of this in an earlier post called The Dorsal Hand: The Dorsal Foot's Better Looking Sibling. The image below demonstrates the location of both extensor digiti mimimi and the last muscle we'll cover today, anconeus.

Location of extensor digiti minimi and anconeus. 

Anconeus is somewhat easy to remember, because it's the shortest muscle in the extensor/supinator group and has the shortest name. It is triangular in shape and can be found running from the lateral epicondyle of the humerus to the proximal end of the ulna. As mentioned in the last post, it can also be found by following the crease along the ulnar side of the arm (the one that divides the two forearm muscle groups) proximally (which means upward, toward the body) until it suddenly ends. When it ends, you'll see the triangular shape of anconeus.

Here is one last image to help you visualize more clearly the four muscles we covered today. This is a repeat from part 1 and shows outlines of all the muscles in the dorsal compartment. Look closely at those we covered today: extensor carpi ulnaris, extensor digitorum, extensor digiti mimini, and anconeus. Ignore the rest for now; we'll get to them soon.

Next time we'll find the last three muscles in the dorsal forearm, those that run along the radial side. They are a little more difficult to spot, but if were start by locating the twin muscles, everything else falls into place pretty clearly.

There are also three muscles at the distal end of the dorsal forearm, but those move the thumb around, and most of what we see of them is their tendons, which show up on the wrist. So we'll cover those in a wrist post. Thanks again to my model, Shannen. See you next time!

Tuesday, July 19, 2011

The Dorsal Forearm, Part 1: Compartment Search

While I'm very happy to get to the first dorsal forearm post, let me first apologize for the long hiatus I've inadvertently taken. In addition to teaching at the American Academy of Art, I am also on the part time faculty at the Biomedical Visualization program at the University of Illinois at Chicago, a fabulous graduate program for future medical illustrators, animators, and anaplastologists. Although it's kept me from my blog for the past few weeks, I've very much enjoyed reading and editing many compelling research papers that this year's graduating BVIS students are finishing up. I can't give away their content, but let's just say it's amazing to see what can be done with MRI data and 3D modeling and animation applications!

The dorsal forearm is one of my favorite lectures, as its intricate and complex musculature is unmatched anywhere else in the body. At the Academy, we devote three entire classes to just the forearm; it deserves a slow and thorough explanation. I can't fit even the dorsal side into one post here, so I'll be dividing it up into a few.

Muscles in the limbs are categorized into groups (which are defined by function) that reside in muscle compartments (which are defined by location.) The forearm has two muscle compartments, the dorsal compartment (in which the extensor/supinator muscle group resides) and the ventral compartment (in which the flexor/pronator muscle group resides.) I wish I could say the dorsal and ventral compartments were placed perfectly dorsally and ventrally on the arm, but alas, they are not. Each spills over onto the other side a bit. This means while most extensor/supinators are seen on the dorsal side of the arm, a few can be seen peeking over onto the ventral side, and while most flexor/pronators are seen on the vental side of the arm, a few can be seen peeking over onto the dorsal side. 

Have I lost you yet? Agh, I know this verbal explanation is really dry, so lets try a diagram. Click for an enlarged view if necessary!

Above we see dorsal (top side) and ventral (underside) views of the right forearm. We also see the location of the extensor/supinator muscle group (shown in blue) and the flexor/pronator muscle group (shown in green.) Notice how we see mostly extensor/supinators on the dorsal side, but a few flexor/pronators creep over there. Similarly, we see mostly flexor/pronators on the ventral side, but a few extensor/supinators creep over there. It's like they each know where their home is, but they can't resist peeking around to the other side.

Since the two forearm muscle groups are not clearly divided between the ventral and dorsal sides, we use other landmarks to find their borders. On the dorsal side of the arm, the crest of the ulna (the distal end of the bone that can be seen and felt on the surface of the arm) is the border between the two muscle groups. On the ventral side, the biceps brachii tendon (coming from the biceps brachii muscle on the anterior upper arm) is the border between the two muscle groups.

When observing the muscles of the forearm, the best way to become oriented is to first figure out where the two muscle groups (and the borders between them) are. As such, when observing the dorsal forearm, the first structure we want to identify is the ulnar crest. This actually doesn't look like much on the surface, but it's easy to find A) because we can usually feel it, and B) because it merges with a nice little furrow at its proximal end.

In the above photograph, the arrow is pointing to the crease along the ulnar crest. Everything above this in the photograph is the extensor/supinator group, and everything below this is the flexor/pronator group.

In this photo, the dashed line follows most of the furrow along the ulna distally until we reach the head of the ulna (the little bump on the pinky side of the wrist. The furrow stops suddenly at its proximal end because at that point we run into the anconeus muscle, a small triangular muscle between the lateral epicondyle of the humerus and the olecranon process of the ulna (what we normally think of as the elbow.)

The area in blue (above the red dashed line) is the extensor/supinator group. The area in green (below the red dashed line) is the flexor/pronator group. Muscles are outlined and labeled:  BRR (brachioradialis); ECRL (extensor carpi radialis longus); ED (extensor digitorum); EDM (extensor digiti minimi); ECU (extensor carpi ulnaris); Anc (anconeus); and FCU (flexor carpi ulnaris) which is the only muscle from the flexor pronator group that can be seen from this view.

In the above image, we can still see the division between the two muscle groups (shown with a red dashed line) but we can also see outlines of muscles within each compartment. Notice how the division line runs right along the ulnar crest and ends distally at the head of the ulna. Notice also how the proximal end of the division line ends at the triangular anconeus muscle.

Now that things are a little more clarified, lets revisit the unembellished arm photo and take another look at the muscles shapes.

So I think that's all we'll cover for today. Keep in mind that these early posts tend to show the body in odd positions because we are still covering straightforward anatomy, and these positions are meant to help pronounce all structures in a given area. Later on we'll observe the arm (and the rest of the body) in more natural poses and note what structures are seen then. 

Dorsal forearm part 2 to come soon! At that point we'll look more closely at the individual muscles and learn to tell them apart. Thanks to my forearm model, Shannen!