Thursday, July 19, 2012

The Head: Part One of Oh My Gosh, Who Knows?

The human head is incredibly complex-- certainly too complex to try to write too much about it in one post. When I took gross anatomy in graduate school, we spent as much time dissecting the human head as we spent on the rest of the body. And for good reason. There's just a heck of a lot going on in there. On this blog, of course, we won't be discussing any of the deep structures, like the brain, the twelve cranial nerves, or the elaborate labrynth of cerebral blood vessels. But in this area of the body, even the surface landmarks are packed in. I would like very much to savor them one post at a time, but before we do this, we she should probably start with a head overview. 

The head differs a great deal from the rest of the body, even landmark-wise, because not much muscle shows here. There are muscles that obscure most of the skull, but they are very thin and flat and, as such, don't show much on the surface. Most of the facial muscles insert into the skin (as opposed to inserting on bone) because it's the skin that they move-- for the sake of facilitating facial expression. But most of the muscles themselves don't show up individually.

Sometimes we can see evidence of two cranial muscles, the temporalis muscle and the masseter muscle, because they contract a bit when we are chewing. The masseter, the thickest muscle on the skull, lies over a portion of the mandible, just anterior to the ear, and sometime even its striations can be seen during chewing. The wide temporalis muscle lies on the side of the head, just above the ear, and sometimes we can see it, too, flexing, when an individual is chewing or clenching. 

Movement of both the temporalis muscle and the masseter muscle can sometimes be seen when an individual is chewing. These are among the only muscles on the head that can be identified individually on the surface.

But other than these occasionally visible muscles, most of the head's prominent surface landmarks are bone. We'll cover each of these in future posts. But before we begin this, let's look at the basic structure of the skull.

The skull has two basic portions, the cranium and the facial region. These are good to keep in mind when drawing the head, because each has its own shape and the two together help define the form of the head. The cranium is the oval shaped, hollow portion of the skull that encases and protects the brain. It shaped somewhat like an egg and is tilted upward anteriorly. The facial region of the skull is the area on which the facial features (such as the eyes, nose, and mouth) rest. It's shaped like a mask and hangs off the front of the cranial egg shape. 

The image below shows a lateral view of the skull alone and with outlines of the cranial and facial regions. The cranium is shown in pink and the facial region is shown in blue.

The cranium is the egg shaped portion of the skull than encases and protects the brain. The cranial region of the skull is outlined in pink here. The facial region, outlined in blue, is the mask shaped portion of the skull. It's the area on which the facial features rest.

It's important to consider the shapes of both these regions when drawing the head. The region shapes may also be used to determine proper head proportions and to position the ear. When measuring any human head from its most anterior to its most posterior point, the halfway point almost always falls just behind the mandible and just in front of the ear. When drawing a head, placing a vertical line halfway back on it helps to properly position both the back edge of the mandible and the front edge the ear. This is demonstrated in the image below.

When divided in half from front to back, the halfway way point on the human head falls just posterior to the mandible and just anterior to the ear. This is helpful to know when placing an ear on a drawing of the head. The black lines here show the anterior and posterior halves of the head. The verticle strip of beige shows where tissue depth has been accounted for on the anterior side (since its greater here than on the posterior side.) The red line shows the placement of the ear behind the middle line.

In upcoming posts we will begin examining the individual bony landmarks of the skull more thoroughly. These include the superciliary crests, the zygomatic arch, the mental protuberance, the angle of the mandible, and the occipital protuberance, to name a few. This will take, well, who knows how long? But I don't mind! Do you? 

1 comment:

  1. Thank you for this --great information and illustration!