Saturday, August 17, 2013

The Anterior Torso: Peel Away The Layers

Hello everyone! I hope you've been enjoying the lazy days of summer. I certainly have— particularly the lazy part. I've been promising for months to continue posting student work from my Advanced Anatomy class, but other pursuits, such as drawing, painting, playing music, swimming, and sitting on the deck staring into space have gotten in the way. Well, today I rectify myself.

You saw Izzy Carranza's clay spine model in The Vertebral Column: Have Some Backbone, and the lovely watercolor work of Jeff Sant in A Beautiful Exaggeration: More Student Forearm Paintings. This time it's Justine Herrera's turn. Justine took my advanced class in the spring of 2012, so this is long overdue. Justine is a lovely, talented and hard working individual whom I've had the pleasure of knowing for several years. You can view more of her work here. For her final Advanced Anatomy assignment, Justine chose to create digital illustrations of the muscles of the anterior neck and torso.

I'm particularly glad to have Justine's permission to use her anterior torso piece here; after almost two and a half years of working on this blog, I have yet to cover that area. So... let's go!

Justine's illustration (below) shows the anterior torso muscles intact. This is not always the case. Often abdominal muscle illustrations show half of the muscles dissected out so more internal layers are exposed. Several layers of muscles and aponeuroses make up the anterior wall of the torso, and often it's in the interest of the viewer to see all of them, as well as their relationships to one another. For our purposes as artists, however, the superficial layers are what most affect the figure's surface appearance. As such, we will stick to those.

A side note, though (and pardon me as my inevitable, undeniable love for terminology once again creeps into my blog)— all the abdominal muscles in the area, including those hidden, have been assigned wonderfully descriptive names: The most superficial muscle on the abdomen (which can be seen here under the milky white, semitransparent rectus sheath) is called the rectus abdominis muscle. The word abdominis refers to the abdomen, and the word rectus means "erect" or "running up and down," which indicates the direction in which the fibers of this muscle run. Deep to this muscle runs another with a similar name. We can't see the tranversus abdominis muscle on the surface of the body, but its name also describes its fiber direction as well as its location on the abdomen; the word transversus means "side to side," the direction in which the fibers of this muscle run.

Let's take a look at Justine's digital illustration of the human anterior torso musculature. Please click on the image to see it at full size. You might even want to open this image in a separate window so you can keep it in front of you while reading the descriptions below.

Let's start by looking at the rectus abdominis muscle a little more closely. You'll notice in Justine's illustration that this muscle is broken up into eight little sections that are divided by thick tendinous lines. These divisions are what give the rectus abdominis muscle its "six-pack" appearance on the body's surface. Of course this six-pack is only visible if there is little adipose (fat) tissue obscuring it. In addition, it's actually an eight-pack! But typically only the six sections superior to the umbilicus (belly button) are seen on the surface, thus giving it more of a six-pack appearance.

There are two different (although very similar looking) types of structures dividing the rectus abdominis muscle into its sections. First we have the linea alba, the long vertical tendon running down the midline of the anterior torse, dividing the rectus abdominis muscle into two bilateral portions. The term linea alba is Latin for "white line." While the linea abla runs the entire length of the rectus abdominis muscle, the portion inferior to the umbilicus is almost never visible on the body's surface; if the linea alba does make a surface appearance, we usually see only the portion superior to the umbilicus. This is because there is typically more adipose tissue over the lower portion of the abdomen.

The interrupting tendons further break the rectus abdominis muscles into sections. These tendons are similar to the linea alba except they run transversely through the muscle, and there are three bilateral sets of them. The interrupting tendons also may show on the surface of the body. Their relative locations are fairly consistent, and as such these lines can be drawn with accuracy using the following guidelines: 1) There are three sets; 2) The most inferior (lowest) set is at or very close to the level of the umbilicus; 3) the most superior (highest) set is at or just below the thoracic arch; 4) the middle set is centered between the upper and lower set, so the three sets are fairly equidistant from one another, and 5) the highest set tends to be the most arched, and the lowest set tends to be the least arched.

On either side of the rectus abdominis muscle we find the external oblique muscles. This muscle is also named for the direction of its fibers, which run at a 45 degree, or oblique, angle. This muscle is called the external oblique because of its relationship to another muscle with oblique fibers, the internal oblique. As the names tell us, the internal oblique muscle also has oblique fibers (although opposite to those of external oblique) and it is deep to the external oblique, meaning its location is more internal on the human body. While the internal oblique is not visible on the body's surface, its external oblique counterpart is. The external oblique muscles cover the sides of abdomen, and while its upper portion isn't all that remarkable in shape, its lower portion is more concretely identifiable. The lower portion of the external oblique muscle attached to the iliac crest (the bony ridge on the lateral hip). Just above this attachment, the external oblique tends to bulge out over the bone, casting a little shadow over the hip. This bulging portion of the external oblique muscle is commonly known as the flank pad. We'll cover this more thoroughly in a pelvis and hip post to come later.

The upper end of the external oblique muscle is fairly flat and nondescript, but we can, in this area, see it intertwining with the serratus anterior muscle. This muscle is named for its serrated (jagged) shape. There is also, as you can surmise from the name, a serratus posterior muscle, which is located on the posterior torso but is not typically visible on the body's surface. The serratus anterior muscle is quite visible on the surface, particular when it's being used to draw the two scapulae anteriorly. This is often the muscle body builders are showing off when they assume their stooped over aarrggh pose. (Think of Saturday Night Live's Hans and Franz.) If you follow the serratus anterior muscle posteriorly, you can see it disappearing under a posterior and lateral torso muscle, latissimus dorsi.

The last muscle we'll look at today is the pectoralis major, which is found on the anterior side of the thoracic cage. The word pectoralis come from pectus, which is Latin for "breast," and the word major tells us that their is also a pectoralis minor muscle, which is smaller than pectoralis major and deep to it, rendering it invisible on the body's surface.

One common mistake artists tend to make when drawing anterior chest muscles is lining up the lower border of the pectoralis major muscle with the thoracic arch. These two structures do not line up! The lower border of the pectoralis major muscle runs approximately 1.5" to 2" superior to the thoracic arch. There is sort of a flat "no man's land" in between the two, where the rectus sheath runs over the lower portion of the thoracic cage. This appears fairly flat and bony on the body's surface, as opposed the more full appearance of the pectoral muscles above.

Let's take a look at how some of these structures look on the surface of a mildly defined body. I've found that many surface anatomy references use extremely defined individuals as examples, and while this is not a bad thing, I think it is also useful to see how these structure look on more of an "average" individual— one not particularly defined or muscular, but with some obvious structures showing.

In the photo below, we can see the basic shapes of the abdominal muscles, plus the umbilicus and a bonus view of a few axillary (armpit) muscles.

One final point to cover: You may be wondering why are there so many layers of muscle and aponeuroses on the anterior wall of the abdomen, and why these muscles have fibers that run in all different directions (rectus, transversus, oblique.) These muscles layers (plus the aponeuroses among them) form a strong, protective wall on a portion of the trunk that needs it most. While the thoracic organs (primarily the heart and lungs) are protected by the thoracic cage, and posterior torso is protected by both the thoracic cage and the vertebral column, the abdomen has no bony protection at all! Odd, considering there are so many important abdominal organs there, including the stomach, the liver and gallbladder, the intestines and the spleen.) The thick, layered wall of abdominal muscles compensates for this lack of bony protection.

Next time we'll look at Justine's other final and equally beautiful illustration for Advanced Anatomy class, that of the anterior neck muscles. Thanks to Justine for letting me use her lovely work. Again, to see more of Justine's beautiful and diverse art, go here!

Until next time!

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