Sunday, May 29, 2011

The External Ear: Shhh, I'm Listening to Reason!

It stands to reason that a structure's design enhances its function. It stands to reason that its size, shape and materials best suit that function. It stands to reason that, in multiples, that structure has latitude for variability while maintaining the same basic parts. And maybe most of all, it stands to reason that the best examples of such a design would be found on the human body.

While most any anatomical structure could provide a satisfying conclusion to this goofy little rumination, I think one of the most interesting and unique examples is the lovely, complex, variable yet constant structure of the human external ear.

When learning to draw any part of the human body, we're taught, of course, to remember what must always be shown, what is constant. But learning those things shouldn't come at the cost of remembering the variations, the ways in which these structures may differ from person to person. While most human ears can be drawn with the same basic parts and pieces, the relative sizes of and relationships among those pieces can vary a great deal. Drawing the ear becomes much easier (and a lot more interesting) when we remember both the constants and the variables. 

The ear is constructed of an outer rim, the helix, and an inner rim, the antihelix. These two structures form sort of an oblong bowl shape. The bottom of that bowl is the concha; this is the deepest point before we get to the external auditory meatus, the opening in the temporal bone that leads to the middle and inner ear structures. On the anterior side of the ear, there is a small notch (the anterior notch) which lies above the tragus, a small flap of cartilage. One of the most common mistakes in rendering the ear is drawing a connection between the helix and the anterior notch. They don't connect there, but it seems their proximity to one another makes us think they should. The difference can be seen in the illustrations below.

The helix actually arises on its leg from the bottom of the concha, usually very gradually. Then it encircles the entire ear and end at the very bottom, at the lobe. The antihelix appear to emerge from underneath the upper portion of the helix, on two legs. Those legs merge together and the antihelix curves around posteriorly as a single unit until it ends at the antitragus (just opposite the tragus, which is what the name antitragus means.)

In addition to names for external ear structures, we also have names for the spaces and depressions among them. We have the intertragical notch, which is the small notch between the tragus and antitragus. We have the scapha, which is the long valley between the helix and the antihelix. And we have the triangular fossa, which is the depression between the two legs of the antihelix.

By the way, scapha is Latin for boat, and it seems that anatomical structures with a depressed center (like a boat) often have this name or some variant of it. We have a scaphoid bone in our wrist which has a depressed shape. Navicular means the same thing, and we have a navicular bone in our foot that also has a depressed shape.

Most of the above mentioned structures are composed of cartilage, which is the ideal material for the human external ear, as it's flexible but it holds its shape. If it were entirely skin and fatty tissue, it would flop over on itself and lose its satellite dish shape that draws sound waves into the auditory meatus. On the other hand, if the external ear was made of bone, it would hold its shape but it would break easily. So most of the external structure is composed of cartilage, although the lobe is mostly fatty tissue, which is why it's floppier than the rest of the ear.

One of the most noticeable variations in external ear anatomy is that of free lobes vs. attached lobes. This is a genetic trait. Although there's not 100% consensus, it's widely believed that attached earlobes are the recessive trait. If you have attached lobes, this means you most likely have two recessive alleles for it.

Another variation among ears is the depths of the sunken areas. The concha, scapha, and triangular fossa range from very shallow to very deep. In addition, the intertragical notch (the small notch between the tragus and the antitragus) varies somewhat in width from one individual to the next.

I would also like to write about rendering the ear and its shapes from a variety of other angles, but this post has gone on long enough! Shall we save it for another day?

It also stands to reason that my lovely ear models deserve a load of thanks! They include family members, neighbors and students: Daniel, Henry B., Henry G, Hillary, Jeff, Nick, Sean, Stephanie, Theresa, and Thomas.

Sunday, May 15, 2011

The Anterior Neck: Theme and Variations

It's difficult to prescribe one exact way to draw the anterior neck because its surface appearance depends on so many variables. Superficial anatomical structures and their degree of visibility change with head position, facial expression, age, body type, and even the level of physical exertion. Today we'll look at these structures and discuss the conditions that affect their visibility.

Typically the easiest landmark to identify first is the jugular notch (a.k.a. suprasternal notch), a small divot on the superior surface of the manubrium of the sternum. On either side of the jugular notch, you will probably see origin tendons of the sternocleidomastoid muscle, as well as the medial ends of each clavicle. One common problem in figure drawings is a jugular notch shown flowing directly into the clavicles. We tend to draw it this way because we know the clavicles connect directly to the manubrium-- but we tend to forget that you can't really see that connection on the surface. Why can't we see it? Because the origin end of the sternocleidomastoid muscle attaches to the manubrium and to the medial ends of the clavicles, and this attachment obscures the connection between the manubrium and the clavicles. The paintings below show the difference.

While the origin end of the sternocleidomastoid muscle (particularly the manubrial attachment) is usually visible as a surface landmark, whether we can see the rest of the muscle depends primarily on head position. We know from its name that sternocleidomastoid originates on the sternum (sterno) and the clavicle (cleido) and inserts on the mastoid process of the temporal bone (mastoid) which is the bony bump you can feel just behind your ear. So if the entire sternocleidomastoid muscle shows, you'll see its split origin end come from the manubrium and the clavicle, you'll see it merge together as it extends superiorly, and finally you'll see it attach just behind the ear. Just behind the ear-- not way behind the ear or under the ear or in front of the ear, all common mistakes in head and neck drawing.

But how do we know whether or not to show the entire sternocleidomastoid muscle? It's also a common mistake to render both sternocleidomastoids from end to end no matter the head position. But the sternocleidomastoid muscles don't always show in their entirity. When they're not in use, the most we'll usually see of them are their manubrial attachments on either side of the jugular notch.

The sternocleidomastoids (herein referred to as SCMs) do show, however, when they're being used, and they have two uses. First, when the SCMs are used together, they hold the head in position when the torso is tilted backward. In other words, you can lean your body backward without your whole head flopping backward because your SCMs, when working together, hold your head in place over the torso. This is shown in the photo below of my lovely neighbor Stephanie.

When used separately, the sternocleidomastoid muscles have another function; they turn the head to the left or to the right. We use the left SCM to turn the head right, and we use the right SCM to turn the head left. As such, if the head is turned to the right, only the left sternocleidomastoid shows, and vice versa. The photo below shows Stephanie with her head turned to her left, so only her right SCM is showing from end to end. The left one is showing, but only at its manubrial attachment.

The SCMs create a sort of V shape on the anterior neck, and there are two more structures inside this V that can often be observed on the surface: First, we have two long thin sternohyoid muscles. As their name suggests, the sternohyoids originate on the sternum (the superior edge of it), extend upward, and insert onto the hyoid bone, a small horseshoe shaped bone in the anterior neck. You can't see the hyoid bone on the surface, but you can just barely feel it if you really dig your fingers into your neck just below the mandible until you're going "gagghhh!"

The sternohyoids run almost parallel, but not quite; they are slightly closer together at their insertion end than at their origin end, as you can see in the photo and illustration below. You usually can't see the sternohyoids in young people, but they begin to appear on the surface in one's 40s. Often neck structures are more visible in older age because fat tissue in the neck dissipates and the skin gets thinner. And often the skin on the neck follows the shape of the sternohyoids more and more with advanced age. So very old individuals will have two long folds down the anterior neck that run right along the sternohyoids.

In between the sternohyoid muscles is another structure that is usually visible on the surface-- the thyroid cartilage, colloquially known as the Adam's apple. The thyroid cartilage is part of the trachea, a cylindrical structure through which air travels from the throat into the lungs. The trachea is made up of a series of cartilaginous rings, bound together by connective tissue. The thyroid cartilage is the most superior ring of tracheal cartilage. It is much larger and more prominent than the rest, and as such, tends to poke out through the neck.

The ring of thyroid cartilage houses the larynx, in which our vocal chords are found. Men typically have deeper voices than women because they have larger larynges and larger vocal chords. This means they also have a larger thyroid cartilage in which to house them. This is why the thyroid cartilage is generally much more visible on a man's neck than on a woman's.

Just below the thyroid cartilage, there is another ring called the cricoid cartilage. This is not as large or prominent as the thyroid cartilage, but it can often be seen on the surface as well.

When observing the anterior neck you might also see the external jugular veins, bilateral vessels that return blood from the head to the heart. In certain situations you might see them popping out on the neck. Each one runs right over the SCM at an oblique angle to it. There's usually no need to show them in a drawing, though, unless your subject is exerting a great deal of physical energy (or is just really mad!)

There is another very thin anterior neck muscle that lies superficial to all of this-- the platysma! But its appearance is fleeting, and I've rambled on long enough now, so we'll leave that for another day.

Bonus question: As I mentioned above, the sternal/clavicular end of the sternocleidomastoid muscle is its origin. The mastoid process of the temporal bone is its insertion. Does anyone know how we know this for sure? If so, post below!

Tuesday, May 3, 2011

The Ventral Forearm: What are those Tendons?

While the ventral side of the forearm is not actually less complicated than the dorsal side, it appears so because few of its muscles show clearly. Compared to the dorsal side of the forearm, the ventral surface is smooth and uncomplicated. The ventral side of a vertebrate is generally considered its "underbelly"-- paler and less hairy than its dorsal counterpart because of fewer hair follicles and less melanin production.

But the distal end of the ventral forearm does have a few prominent surface landmarks. I write "a few" instead of a specific number because the number depends on the individual; one of the tendons that's often seen at this location is actually missing in 12-15% of the human population!

Let's back up a little. In most cases, there are two fairly visible tendons down the approximate center of the ventral wrist. (Their degree of visibility also depends on genetics, hand position, and temporary body variations such as those seen in water retention.) Those two tendons come from the palmaris longus muscle and the flexor carpi radialis muscle. They are shown in the illustration below. We can tell this is a ventral view of the forearm because we can see the palmar aponeurosis (a thin, tendinous sheath that is only on the palmar side of the hand) and because, um... there are no fingernails!

If both the flexor carpi radialis tendon and the palmaris long tendon are visible, it's easy to tell one from the other; the flexor carpi radialis tendon is more toward the radial (thumb) side of the arm, as its name implies. Once that's been established, one can deduce that the other tendon is most likely that of palmaris longus.

The palmaris longus tendon is also more superficial than the flexor carpi radialis tendon because it runs outside the annular ligament, while all the other wrist tendons run deep to it. The annular ligament is a ring-like ligament ("annular" is Latin for "ring-like") that wraps around the wrist like a bracelet and retains the position of the tendons that run from the forearm and into the hand. After surpassing the annular ligament, the palmaris longus tendon inserts directly into the palmar aponeurosis and tenses it to help strengthen the grip.

In some cases, one or both of these tendons are difficult to see, especially when the hand is relaxed. Because both of these muscles are flexors, you can force them to stand out more by flexing the wrist and tensing the hand into sort of a claw shape.

In the photo below, you can easily see both ventral forearm tendons and tell them apart. First of all, flexor carpi radialis is the more radial of the two (meaning it is closer to the thumb side.) Second, palmaris longus can be seen more clearly where the wrist meets the hand, because at that point, flexor carpi radialis is traveling under the annular ligament and palmaris longus is not.

But because palmaris longus is missing in 12 to 15% of the human population, we must also consider what the wrist looks like if there is no palmaris longus muscle. If the palmaris longus muscle is missing, you can usually still see the tendon of flexor carpi radialis. Ulnar to that, where the palmaris longus tendon would normally be seen, you might see nothing but smooth skin. Or you may see some less pronounced tendons, which would most likely be those of flexor digitorum superficialis, which lies deep to everything we've talked about so far. Flexor digitorum superficialis is Latin for "superficial flexor of the fingers," which implies there is also a deep flexor of the fingers (flexor digitorum profundus.) But that muscle is very deep and there is usually no evidence of it on the surface.

In my Anatomy class at the American Academy of Art in Chicago, we always end week 11 ("forearm week") with all my students making claw hands and checking to see whether they have the palmaris longus muscle. As expected, most students have it, a few don't, and occasionally one or two have palmaris longus in one forearm but not the other. But there is one occurrance of which I've found I can be almost 100% sure: Those students who do not have the palmaris longus tendon always seem bothered by this fact, and some actually refer to themselves as freaks! Let's make this clear once and for all: You are not a freak if you don't have the palmaris longus muscle! Distinctive, yes, but not a freak.

Do you have the palmaris longus muscle? See if you can tell and let me know. I could use some photos of forearms lacking the palmaris longus tendon, so if you don't think you have it, please send a shot!