Thursday, July 28, 2011

The Dorsal Forearm, Part 2: Which Side Are You On, Anyway?

Now that we’ve established a method for identifying the two compartments of the human forearm, let’s look at the dorsal compartment a little more closely. The dorsal side of the forearm is the "top" side-- the side that usually faces upward and is generally darker and hairier (due to more melanin and a greater number of hair follicles.) The muscles on the dorsal side of the arm (unlike those on the ventral side) are arranged in a single layer, so they all lie directly under the surface of the skin. This means most of them can be seen pretty clearly on a well defined individual, although their relative visibility will depend on the position of the hand. The muscles in this compartment are all part of the extensor/supinator muscle group, which means they either extend the wrist or fingers or supinate the forearm, or both.

When considering which muscles will show in a given position, we want to keep in mind what the muscles in a specific area tend to do. We are looking at the extensor/supinator group today, so it's safe to assume that these muscles can be seen more clearly when we are supinating the forearm (which means turning the palm upward) or extending the wrist or fingers (which means opening them up.) It's also helpful to remember that muscles on the radial (thumb) side of the arm tend to pull the hand in the direction of the radius (or abduct it) and muscles on the ulnar (pinky) side of the hand tend to pull the hand in the direction of the ulna (or adduct it.) As such, radial side muscles tend to show more when the hand is abducted and ulnar side muscles tend to show more when the hand is adducted. Knowing these four movements will help us to remember which hand positions will make which muscles stand out.

So... here is an overview of what we'll cover today:

When identifying muscle shapes on the forearm (or anywhere else on the body, for that matter) it’s best to locate the most obvious structures first and then find the other structures based on their relationships to the former. Today we'll look at the ulnar (pinky) side muscles, because I think they're easier to find first. Once we've established their location, we'll go on to the radial (thumb) side in the next post.

When becoming oriented on the dorsal forearm, I usually begin with what I like to call the twin muscles-- two muscles the seem to look more alike than any of the others. They are often the first to make themselves evident when looking at the dorsal forearm. The twin muscles are extensor digitorum and extensor carpi ulnaris. They both originate at the lateral epicondyle of the humerus (the bony bump on the lateral side of the elbow) they are about the same width, and they both run straight down the dorsal side of the forearm, toward the hand. Find them below in both the anatomical rendering and the photograph, and notice how similar they are.

The dorsal forearm "twin" muscles, extensor digitorum and extensor carpi ulnaris, look very much alike. We can distinguish one from the other by remembering that extensor carpi ulnaris is closer to the ulnar (pinky) side of the arm. 

Please note that the elbow position in the photo does not exactly match that in the diagram. But for our purposes, we can see what we need to see: The twin muscles and the furrow where the two muscle groups meet (shown with a dashed line.)

OK, so now that we’ve found the twin muscles, the next step is to distinguish them from one another. This is easy. The twin muscles are extensor digitorum and extensor carpi ulnaris. It makes sense that, of the two, extensor carpi ulnaris is closer to the ulnar side of arm. It actually lies right next to the ulna (whose location is designated by the furrow on the forearm, marked with a dashed line in the photo.) Once we've positively identified extensor carpi ulnaris, we can assume the other twin must be extensor digitorum.

The next muscle we'll locate is extensor digiti minimi. As its name tells us, this muscle extends the small finger (digiti minimi is Latin for smallest finger.) Extensor digiti minimi is very easy to locate because it's very narrow and lies right between the twin muscles. On the anatomical diagram, you can see that the tendon of this muscle is headed straight for the smallest digit, where it will insert on its dorsal side. This tendon is often visible on the dorsal hand. You can see an example of this in an earlier post called The Dorsal Hand: The Dorsal Foot's Better Looking Sibling. The image below demonstrates the location of both extensor digiti mimimi and the last muscle we'll cover today, anconeus.

Location of extensor digiti minimi and anconeus. 

Anconeus is somewhat easy to remember, because it's the shortest muscle in the extensor/supinator group and has the shortest name. It is triangular in shape and can be found running from the lateral epicondyle of the humerus to the proximal end of the ulna. As mentioned in the last post, it can also be found by following the crease along the ulnar side of the arm (the one that divides the two forearm muscle groups) proximally (which means upward, toward the body) until it suddenly ends. When it ends, you'll see the triangular shape of anconeus.

Here is one last image to help you visualize more clearly the four muscles we covered today. This is a repeat from part 1 and shows outlines of all the muscles in the dorsal compartment. Look closely at those we covered today: extensor carpi ulnaris, extensor digitorum, extensor digiti mimini, and anconeus. Ignore the rest for now; we'll get to them soon.

Next time we'll find the last three muscles in the dorsal forearm, those that run along the radial side. They are a little more difficult to spot, but if were start by locating the twin muscles, everything else falls into place pretty clearly.

There are also three muscles at the distal end of the dorsal forearm, but those move the thumb around, and most of what we see of them is their tendons, which show up on the wrist. So we'll cover those in a wrist post. Thanks again to my model, Shannen. See you next time!

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