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.

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