Sunday, May 15, 2011

The Anterior Neck: Theme and Variations

It's difficult to prescribe one exact way to draw the anterior neck because its surface appearance depends on so many variables. Superficial anatomical structures and their degree of visibility change with head position, facial expression, age, body type, and even the level of physical exertion. Today we'll look at these structures and discuss the conditions that affect their visibility.

Typically the easiest landmark to identify first is the jugular notch (a.k.a. suprasternal notch), a small divot on the superior surface of the manubrium of the sternum. On either side of the jugular notch, you will probably see origin tendons of the sternocleidomastoid muscle, as well as the medial ends of each clavicle. One common problem in figure drawings is a jugular notch shown flowing directly into the clavicles. We tend to draw it this way because we know the clavicles connect directly to the manubrium-- but we tend to forget that you can't really see that connection on the surface. Why can't we see it? Because the origin end of the sternocleidomastoid muscle attaches to the manubrium and to the medial ends of the clavicles, and this attachment obscures the connection between the manubrium and the clavicles. The paintings below show the difference.




While the origin end of the sternocleidomastoid muscle (particularly the manubrial attachment) is usually visible as a surface landmark, whether we can see the rest of the muscle depends primarily on head position. We know from its name that sternocleidomastoid originates on the sternum (sterno) and the clavicle (cleido) and inserts on the mastoid process of the temporal bone (mastoid) which is the bony bump you can feel just behind your ear. So if the entire sternocleidomastoid muscle shows, you'll see its split origin end come from the manubrium and the clavicle, you'll see it merge together as it extends superiorly, and finally you'll see it attach just behind the ear. Just behind the ear-- not way behind the ear or under the ear or in front of the ear, all common mistakes in head and neck drawing.

But how do we know whether or not to show the entire sternocleidomastoid muscle? It's also a common mistake to render both sternocleidomastoids from end to end no matter the head position. But the sternocleidomastoid muscles don't always show in their entirity. When they're not in use, the most we'll usually see of them are their manubrial attachments on either side of the jugular notch.

The sternocleidomastoids (herein referred to as SCMs) do show, however, when they're being used, and they have two uses. First, when the SCMs are used together, they hold the head in position when the torso is tilted backward. In other words, you can lean your body backward without your whole head flopping backward because your SCMs, when working together, hold your head in place over the torso. This is shown in the photo below of my lovely neighbor Stephanie.





When used separately, the sternocleidomastoid muscles have another function; they turn the head to the left or to the right. We use the left SCM to turn the head right, and we use the right SCM to turn the head left. As such, if the head is turned to the right, only the left sternocleidomastoid shows, and vice versa. The photo below shows Stephanie with her head turned to her left, so only her right SCM is showing from end to end. The left one is showing, but only at its manubrial attachment.




The SCMs create a sort of V shape on the anterior neck, and there are two more structures inside this V that can often be observed on the surface: First, we have two long thin sternohyoid muscles. As their name suggests, the sternohyoids originate on the sternum (the superior edge of it), extend upward, and insert onto the hyoid bone, a small horseshoe shaped bone in the anterior neck. You can't see the hyoid bone on the surface, but you can just barely feel it if you really dig your fingers into your neck just below the mandible until you're going "gagghhh!"

The sternohyoids run almost parallel, but not quite; they are slightly closer together at their insertion end than at their origin end, as you can see in the photo and illustration below. You usually can't see the sternohyoids in young people, but they begin to appear on the surface in one's 40s. Often neck structures are more visible in older age because fat tissue in the neck dissipates and the skin gets thinner. And often the skin on the neck follows the shape of the sternohyoids more and more with advanced age. So very old individuals will have two long folds down the anterior neck that run right along the sternohyoids.



In between the sternohyoid muscles is another structure that is usually visible on the surface-- the thyroid cartilage, colloquially known as the Adam's apple. The thyroid cartilage is part of the trachea, a cylindrical structure through which air travels from the throat into the lungs. The trachea is made up of a series of cartilaginous rings, bound together by connective tissue. The thyroid cartilage is the most superior ring of tracheal cartilage. It is much larger and more prominent than the rest, and as such, tends to poke out through the neck.

The ring of thyroid cartilage houses the larynx, in which our vocal chords are found. Men typically have deeper voices than women because they have larger larynges and larger vocal chords. This means they also have a larger thyroid cartilage in which to house them. This is why the thyroid cartilage is generally much more visible on a man's neck than on a woman's.

Just below the thyroid cartilage, there is another ring called the cricoid cartilage. This is not as large or prominent as the thyroid cartilage, but it can often be seen on the surface as well.

When observing the anterior neck you might also see the external jugular veins, bilateral vessels that return blood from the head to the heart. In certain situations you might see them popping out on the neck. Each one runs right over the SCM at an oblique angle to it. There's usually no need to show them in a drawing, though, unless your subject is exerting a great deal of physical energy (or is just really mad!)

There is another very thin anterior neck muscle that lies superficial to all of this-- the platysma! But its appearance is fleeting, and I've rambled on long enough now, so we'll leave that for another day.

Bonus question: As I mentioned above, the sternal/clavicular end of the sternocleidomastoid muscle is its origin. The mastoid process of the temporal bone is its insertion. Does anyone know how we know this for sure? If so, post below!

6 comments:

  1. Wow, what an fabulously informative and generous blog! Its like the anatomy class that I never had at school. This is a great source for information. Thanks. MBK

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  2. I agree. This blog is really helpful. It's great to refresh your memory on anatomical facts most of all since it's such a vast and complex subject. Thanks for the info!

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  3. Thanks very much for the kind words! It's good to know someone is reading and finding it useful.

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  4. (I'm finding it useful, too. I'm in Sarah Kaiser's Evanston class and she recommended this site. Thank you! -Natalie W.)

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  5. I was reading Robert Henri's amazing book (1923) The Art Spirit. I came across a reference to the sterno-clido and had to look it up! I'm with Mat, above, grateful for this "instant anatomy class." Thoughtful/generous and hugely helpful.

    THANK YOU!

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    1. Thank you so much for the kind words. I'm glad you are enjoying the blog and finding it helpful!

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