Friday, December 16, 2011

Anatomical Terminology 101: Baby Needs Some Direction

This blog is seven months old now and has slowly and imperceptibly transitioned from its infancy to its... um... toddlerhood? Not sure how to complete that metaphor except to say that my baby is beginning to stand on its own two feet and get around by itself. It's getting hits from all over the world without my having to hold its hand! Ah, now I'm all misty. (ahem) OK, in any case, now that we're standing on our own, maybe this is a good time to go over some basic anatomical terminology that describes overall  direction and location on this newly upright body. 

The first day of my Anatomy course at the American Academy of Art is typically spent going over direction and location terminology-- words that allow us to describes positions of anatomical structures and their relationships to one another without having to rely on pointing. This can be useful when visuals aren't an option. In addition, it's more accurate (not to mention more eloquent) to describe a structure as residing "on the distal end of the ulnar forearm" than "way down on the end of the arm, on the side by the pinky finger."

Let's start with a look at the body's midline, which is an imaginary line that runs down center of the body from either a front or back view; it's a line on either side of which we are basically symmetrical. While no one's body is perfectly symmetrical, most of us have the same basic form mirrored on either side of the midline: We have two ears, two eyes, two arms, etc. As such, there is no midline running down the side of the body, as the front of our body is different from the back.

The above image is an anterior view of the human body. The anterior side of the body is basically the front-- everything from the forehead down to the toes. The back of the body is known as it posterior side.  The posterior side includes everything from the back of the head down to the heels.

Our next two words, medial and lateral, are based on the concept of the midline. Medial and lateral are comparative terms that define a structure's location relative to the midline. A structure that is more medial is closer to the midline, and a structure that is more lateral is farther from the midline. So we might say that the corner of the mouth falls about two centimeters lateral to the midline. (Of course, every structure is lateral to the midline, because the midline is as medial as we can get.)

These terms occur more commonly in anatomical structures that come in pairs, such as the medial and lateral epicondyles of the humerus bone. The epicondyles of the humerus are bumps on its distal end, which is the end of the humerus near the elbow. (The terms distal and proximal will be explained later in this post!) The medial and lateral epicondyles are named as such because they are similar structures that need to be distinguished from one another. This happens a lot in human anatomy-- two or more structures will be very similar but not quite the same, so their names will be similar, too, but with one distinguishing qualifier. In the case of the epicondyles of the humerus, the qualifiers are medial and lateral. We have a medial epicondyle, which is the one closer to the midline, and a lateral epicondyle, which is the one one farther from it. 

These terms are used elsewhere in the body as well, such as the medial and lateral malleoli on the ankles (bumps on the tibia fibula) and the medial and lateral canthi (corners) of the eye. Now that you've read the last paragraph, you should be able to tell which is which.

You'll notice I also pointed out the terms anterior and posterior in the image caption above. The anterior side of the body is basically the front-- everything from the forehead down to the toes. The posterior side of the body is the back-- everything from the back of the head down to the heels. Both images in this post show anterior views of the human body.

Two more terms, superior and inferior, define relative position on the body in a different way. The superior end of the human body is the top of the head, and the inferior end is the bottoms of the feet. So a structure that is more superior is closer to the head, and a structure that is more inferior is closer to the feet. One example of the use of these terms is in the structure names superior vena cava and inferior vena cava. The vena cavae are the largest (and most cavernous) veins in the human circulatory system. They are named superior and inferior because one drains into the top of the heart (superior to it) and the other drains into the heart's underside (inferior to it.) We also use the roots supra- and infra- within anatomical terms to define relative structure positions. For example, the supraclavicular fossa is a depression in the skin just above the clavicle, and the infraorbital foramen is a hole in the maxilla just inferior to the orbit.

The last two direction and location terms we'll cover today are proximal and distal. These words describe relative location on a limb. We can't use superior and inferior for this, because that would change depending on the position of the limb. So we use a terms that don't depend on that: No matter what the position of the limb, there is always one end closer to the torso and one end farther from it. The proximal end of a limb is that closer to the torso. (The root prox- means near, and we see it in other terms such as approximately and proximity.) The distal end of a limb is farther from the torso. (The root dist- means farther or more distant.) So the proximal end of the arm is up by the shoulder, and the distal end of the arm is the fingertips. The leg's proximal and is up by the hip, and its distal end is at the toes.

We can also apply proximal and distal to individual structures in the limbs. For example, the femur has a proximal and and a distal end. And the tibia has a proximal end and a distal end. And we'd describe their relationship by stating that the distal and of the femur articulates with the proximal end of the tibia. 

Well, I've had enough of this, how about you? There are plenty more direction and location terms to cover, but let's save it for later. I'm thinking about getting a babysitter and skipping ahead to a shoulder post next... which would be the proximal end of the arm, right? See you next time.

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