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!

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.

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 CompleteMartialArts.com. Thanks!