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!

Posterior Torso Muscles, Part 2: Under the Radar

You may have noticed that I keep describing the forearm as the most difficult part of human anatomy to teach (and the most difficult to understand!) This area is so difficult for some pretty straightforward reasons: 1) There are so many forearm muscles, and 2) they all look very much alike. But sometimes I forget that the posterior torso muscles, although they are large and varied, are also among the most difficult to understand. In my class, they sometimes require as many days to get through as the forearm muscles. But the posterior torso muscles are confusing for a completely different reason-- unlike most muscles that concern the figure artist, they like to live under the radar.

Like most areas of the body, the posterior torso has several layers of muscle. When rendering most areas of the body, we artists tend to think primarily about the superficial muscle layers-- the ones that are visible at the surface and contribute most to the body's outward appearance. But things are a little different on the posterior torso. Its muscles are exceptionally broad, thin, and flat, which means we sometimes can see deeper muscles showing right through them. This is pretty cool, but it does make things a little more confusing.

In fact, sometimes the deeper muscles of the posterior torso show more clearly on the surface than the superficial muscles. How can that be? This happens because the superficial muscles are often so thin and flat that they just don't look like much on the surface. But the muscles underneath them are rounder and more defined, so it's actually easier to pick out their shapes. 

There are some wonderful examples of this in the figure renderings of Brian Skol. Brian is a student at the Ravenswood Atelier in Chicago, and during his time there he has developed a solid, stunning figure drawing technique and turned out an impressive collection of work. Brian also took my Anatomy course at AAA several years back, and while he is among several top students who mastered the class, he is distinctive in that he's the only one who's ever said the Anatomy final exam was not difficult enough! This comment was unexpected and refreshing, as I'm used to hearing just the opposite.

Several of Brian's figure drawings lovingly demonstrate the posterior torso landscape, but today (and in a few upcoming posts) we'll concentrate on this one. First let's look at the subtle ridges that help define the structure of the back.

Each of the skin ridges in Brian's figure drawing subtly reveals a muscular relationship on the posterolateral torso. Ridge 1 shows the point at which the latissimus dorsi meets the lumbar sheath. Ridge 2 demonstrates the furrow at which the external oblique muscle meets the rectus abdominis muscle. Ridge 3 shows where the serratus anterior muscle travels back toward its insertion on the underside of the scapula. (It's actually under latissimus dorsi at this point, but we can still see it because latissimus dorsi is so thin.) Ridge 4 shows the teres major muscle as it extends laterally toward the upper arm, where it will insert on the anterior side of the proximal humerus. The lower edge of teres major is hidden under latissimus dorsi, but it still shows. Finally, ridge 5 shows the posterior edge of the deltoid.

I know, I know. I did say subtle, right? But part of the beauty of the human body is that so much of it is a mystery; its structures are always there, doing their job, making things happen, but many don't necessarily need to be right in your face; they'd prefer to stay under the radar and let you discover them. 

So let's take a closer look at these shy little devils. The first step in this process was to block in some overall muscle shapes and a few bony landmarks, then to identify them.

Drawing in these basic structure shapes was the starting point for exploring each of the posterior torso ridges more closely. The iliac crest is shown in yellow but unlabeled. The large posterior torso muscles are trapezius (T) and latissimus dorsi (LD). Together with deltoid (D), they form a triangular window, in which we can see infraspinatus (Inf) and teres major (TM). Teres minor is barely peeking out between infraspinatus and teres major, but I didn't label it. Just anterior to latissimus dorsi, we can see two anterior torso muscles: external oblique (EO) and rectus abdominis (RA). I've also included some structures just under the iliac crest: Gluteus maximus (Gmax), gluteus medius (Gmed),  and tensor fasciae latae (TFL) all surround the greater trochanter of the femur (GT). Distal to that we can see vastus lateralis (VL) and the iliotibial band (ITB).

Now let's take a look at the ridges that are so beautifully and accurately rendered in Brian's drawing. We'll start with ridge number 1, where the lumbar sheath meets the latissimus dorsi muscle. First, here's how these structures look from a direct posterior view.

The lumbar sheath is a flat, diamond shaped aponeurosis covering a large portion of the lower back. It serves at an origin point for the large, bilateral latissimus dorsi muscles. We can often see a ridge along the lower back where these two structures meet. Note that both latissimus dorsi and the lumbar sheath have been removed on the right side of the body in this image.

You might recall this image from my first posterior torso post. At that time we looked at the relationships of the larger back muscles and the small triangular window they form (in which we can see some smaller muscles on the posterior surface of the scapula.) Now let's use this image to look at the landscape of the lower back. Occupying most of its space is the lumbar sheath, a diamond shaped aponeurosis (which is broad, flat, tendinous muscle covering.) The lumbar sheath serves as an origin point for the large bilateral latissimus dorsi muscles that run across the lower back and sides. Because the lumbar sheath is not muscle tissue, doesn't bulk up with use. So it can appear somewhat flat in comparison to the lumbar sheath. This means we can often see a ridge where these two structures meet. That's what we're seeing on the area labeled number 1 on Brian's drawing.

In this next image, the basic shapes of latissimus dorsi and the lumbar sheath are placed on top of Brian's rendering. This shows the placement of the individual structures and their meeting point. 

The lumbar sheath (shown in white) is an origin point for latissimus dorsi (LD). Because the lumbar sheath is flat and the latissumus dorsi has some bulk, we can often see a ridge on the back where the two meet. This ridge forms one of the upper edges of the lumbar sheath. The lower edges are formed by a small corner of the external oblique that reaches around to the back and attaches to the iliac crest.

We can see here that both the latissimus dorsi muscle and the external oblique form borders around the lumbar sheath. The angles of these borders give the lumbar sheath its diamond shape. On a more muscular individual, the flat lumbar sheath would stand out more obviously against the surrounding muscles and would become even more visible.

I am looking forwarding to explaining the other four ridges that can be seen in this lovely rendering, but this is getting a little long, so I'm going to sign off for now. I will cover ridges 2 through 5 in an upcoming post, and I'm also working on the rest of the elbow joint posts. See you soon!