Human Anatomy Lesson 1
Lesson 1: Anatomical Terminology, Orientation and Movement
This article is the first lesson in a semester-long undergraduate course in Human Anatomy. I am currently teaching this course at Benedictine University in Lisle, IL. Undergraduate students in my course can refer to the article to refresh their memory about what I said in lecture, and to provide additional or supplementary information. You don't have to be in my course, though, to enjoy the benefits of this lesson!
Learning Objectives : By the end of this lesson, you should be able to ...
- Understand anatomical position, which is the basis for positional terminology in human anatomy
- Use standard directional terminology to describe the position of structures in the human body
- Understand the relational nature of such terminology
- Describe how anatomical planes section the body and why this is important for medical imaging
- Use anatomical position, positional terminology and anatomical planes to describe simple movements of the body and limbs
- Understand the difference between anatomical terminology as applied to humans and non-human animals
- Appreciate how the language of anatomy is contingent on historical factors and evolving medical needs
The first thing we need to do is to develop a language that we can use to talk to one another. Anatomical structures are positioned in particular relations with one another, and we need to know how to discuss this. This is not as trivial a matter as it might at first seem. For example, if I say to you, "your liver is on the right," what do I mean? Is that "your" right? or "my" right? What does it mean if I say "your brain is above your intestines," but you are lying down? And how do we refer to structures that are inside other structures, structures in the feet or hands, or the position of teeth in the tooth row? Obviously, we need to have a common language to discuss these issues.
Orientation of structures in the body is based on anatomical position. Imagine you are standing straight up, eyes and feet facing forward, with your arms at your sides and your palms facing forward. This is anatomical position. All directional terms are oriented relative to the body in this position. So, we can immediately answer a few of the above questions. "Right" and "left" are in relation to this body, so, if you are standing in anatomical position, your liver is on "your" right. In other words, it is the same system followed in baseball: if you are the batter, when you face the diamond right field is to your right, and left field is to your left. In anatomical position, your brain is indeed superior to your intestines, and your intestines are superior to your feet. This is true even if you are lying down. You just have to imagine yourself in anatomical position.
Now that I have introduced anatomical position, we can talk about some of the directional terms that relate to this position. Let's start with two terms that I have already introduced, superior and inferior. "Superior" is up and "inferior" is down when you are standing in anatomical position. As noted above, the brain is superior to the intestines. It is important at this point to realize that these terms are relational, not exact. "Inferior" is not a property of the intestines — the intestines are inferior to some structures, like the brain or structures in the head and neck, but superior relative to other structures in the hip, thigh, leg, and foot.
"Superior" and "inferior" are terms that use anatomical position to describe the relationship between structures in the body. There are, of course, other such terms. Medial and lateral, for example, describe the relationship between structures relative to the midline of the body. Medially-placed structures are closer to the midline of the body, whereas those that are laterally-placed are away from the midline, again in anatomical position. As with superior and inferior, these terms are relational, so that the pinkie finger, for instance, is medial relative to the thumb, but lateral relative to the belly button, which is in the midline of the body.
So far I have described relational terms that can be used to describe the relationship between structures in the "x" and "y" directions (up and down and side to side). But what if you want to describe the position of structures in the "z" direction, i.e., front to back? For that purpose, we can use the terms anterior and posterior. "Anterior" refers to things toward the front of the body, and "posterior" refers to things toward the back of the body. The belly button is anterior to the tailbone. Again, these terms are relational, so that the pinkie finger is anterior relative to the tail bone but posterior relative to the belly button (by the way, you will learn the anatomical names for the pinkie finger, belly button, and tail bone in this course).
The above three sets of terms are the ones most commonly used to orient the body in space and to identify and orient different parts of the body relative to one another. One other set of terms, however, is important so that we are able to refer to parts of the body at different surface levels. In this regard, we use the term "superficial" to refer to structures closer to the outside of the body, and "deep" to refer to structures closer to the interior. Again, these structures are relational. The skin is superficial relative to the surface of the heart, and the surface of the heart is superficial relative to the heart valves.
So far, we have discussed terms that refer to the relationship of structures in the body as a whole. However, sometimes we just want to refer to structures in one part of the body, like the limbs or the head.
For the limbs, there are two relational terms that are very important: proximal and distal. "Proximal" refers to structures toward the midline along the limb, and "distal" refers to structures away from the midline. Again, these terms are relational, so that the elbow is distal relative to the shoulder but proximal relative to the wrist and hand.
The other terms I need to go over all concern structures in the head related to the dental arch and teeth. We need special terms to describe relationships in the dental arch because of its unique parabolic ("U") shape. [If you are in my spring 2016 Human Anatomy course at Benedictine University, you can skip this section for now, since we'll revisit anatomical terminology related to the teeth in the section on the head near the end of the course].
Mesial is the term used to refer to teeth or cusps of teeth toward the front of the mouth/dental arch, and distal is the term for teeth or cusps of teeth toward the back of the mouth. The canine teeth (the ones that are long in vampires), for example, are distal relative to the incisor teeth (the blade-like teeth you use to bite into an apple), but mesial relative to the molars (the teeth you use to chew steak).
You can use the terms "mesial' and "distal" to refer to the position of cusps on the teeth, but these terms are generally used to refer to the position of teeth in the tooth row. The next two pairs of terms, however, are commonly used only to refer to the position of cusps on the teeth, or of parts of the teeth relative to one another. Labial and lingual refer to the front and back of the incisor teeth in particular ("labial" is from the Latin word for lips and "lingual" is from the Latin word for tongue, if that helps you remember it), and buccal and lingual refer to the cheek-side (buccal) and tongue-side of the molars and premolars specifically.
Before you can apply your new knowledge of anatomical position and directional terminology to refer to movements of the body and limbs, it is necessary to describe three anatomical planes in which movement can occur.
The coronal plane cuts the body into anterior and posterior portions. If I wave my arms in the air as if I am on a desert island and hailing a plane, then I am moving my arms in the coronal plane.
The sagittal plane cuts the body into medial and lateral portions. If I do the Running Man, I am moving my arms and legs in the sagittal plane.
The transverse plane cuts the body into superior and inferior portions. If I am a football referee and I signal "no down!" I am moving my arms in the transverse plane. An image made in this plane is often referred to as an axial image or, more colloquially, as a cross-section.
These three anatomical planes are important not only as part of the terminology which we use to describe movement of the body and limbs through space, but also as a basis for how the body is imaged. Radiologists and other imaging technicians regularly "cut" the body into transverse, sagittal, and coronal views in order to see internal structures and diagnose pathologies and diseases. It is very important that you get used to seeing these different views of the body. We will discuss medical imaging technologies in lesson 2.
We now have all the terminology we need to describe motions of the body and limbs through space.
When I do the Running Man, i.e., when I move my limbs in the sagittal plane, I am flexing and extending my limbs. Flexion and extension are movements that occur at joints. Doing the Running Man, I am flexing and extending my arm at the shoulder joint, my forearm at the elbow joint, my thigh at the hip joint, and my leg at the knee joint. "Flexion" is the movement of a limb segment that decreases the angle between two body parts or limb segments at a joint (so, for example, I bring my forearm into closer approximation to my arm or my leg into closer approximation with my thigh), whereas "extension" increases the angle between two body parts or limb segments. Note that flexion of the forearm at the elbow joint and of the leg at the knee joint occur on opposite sides of the body, but both movements are still called flexion.
Abduction and adduction are movements of the limbs and body in the coronal plane, like the example where I am waving my arms on a desert island to signal to a plane. AB-duction is the movement of a limb segment away from the body, whereas AD-duction is the movement of a limb segment toward the body. In AD-duction, you are "adding" the limb segment to your body, if that helps you remember the distinction (also, "ab" is from the Latin word for "away from," in the unlikely event that that will help you remember). In AB- and AD-duction, it can be the limb that is moving, or the body. So, for example, you would be AB-ducting your leg if you were to move it away from your body, but it is also AB-duction if you were to plant your leg on the ground and bend your hip to the side so that your body is parallel to the ground.
Rotation is a term that refers to movement of any part of the body relative to a supero-inferior axis. The head rotates on the neck so that your face can "face" to the right or the left. In general, lateral (or "external") rotation is where the limb segment moves so that the anterior aspect faces laterally, and medial ("internal") rotation is movement of the anterior surface of the limb segment medially.
Pronation refers to rotation of the hand and forearm where the thumb is rotated from its anatomical position starting point around so that the back of the hand faces anteriorly. Once we go over the bones of the body, you will see that pronation is rotating the lateral bone of the forearm, the radius, so that it crosses the medial bone of the forearm, the ulna, which is locked in place. Supination is the opposite motion, where the thumb and radius rotate back around the pinkie and ulna to return to anatomical position. In anatomical position, the thumb and radius are naturally supinated.
Eversion refers to a movement of the foot which positions the sole of the foot laterally, away from the midline of the body. Inversion refers to a movement of the foot which positions the sole so that it is facing the midline of the body. Pronation and supination of the foot are said to occur when the foot is planted on the ground. Pronation, in this case, is a movement similar to eversion (i.e., the ankle "rolls" toward the midline of the body, and the sole of the foot faces laterally) and supination is similar to inversion. If you are a runner you know exactly what I'm talking about.
Circumduction is a complex compound movement at several joints in the body, most notably at the shoulder joint, wrist joint, hip joint, and, to a lesser extent, the ankle joint. At the shoulder and hip joints, circumduction consists of flexion, extension, abduction, adduction, and rotation when you swing your arms or legs in a circular motion next to your body. Circumduction at the ankle joint involves some more specialized terminology, so we will save that for later lessons.
Terminology associated with the spine is a little different, and somewhat illogical in relation to the system we've established (it's just one of those exceptions to the rule that you'll need to learn). Bending of the spine in the coronal plane is known as lateral flexion, not abduction as you might expect.
Non-human Anatomical Terms
Before I end this lesson I would be remiss if I neglected to tell you the difference between anatomical position and terminology as applied to humans and to other animals. We are a unique species in that we are bipedal, or, in other words, we habitually stand and walk around on two limbs instead of four. Because of this, our terminology is centered around an anatomical position where we are facing forward in UPRIGHT position. Animals that walk on four limbs instead of two need to be described using a terminology that is not dependent on an upright anatomical position.
In this case, cranial and caudal replace "superior" and "inferior," and ventral and dorsal (or, in some cases, rostral) replace "anterior" and "posterior."
These terms are, of course, important for comparative anatomy, but they also have their place in human anatomy. You will see later in this course that these terms are applied to directions in the brain and spinal cord. You will also see the terms ventral and dorsal used in many anatomy atlases and textbooks [note for students in my spring 2015 Human Anatomy course at Benedictine University: your book uses the terms "anterior" and "posterior," not "ventral" and "dorsal"].
The Language of Anatomy
In anatomy, you will be exposed to many words and phrases that will not be familiar to you. There are many reasons for this. First, many anatomical terms are from Latin, Greek, or Arabic. If you have taken one of these languages, or even if you have some knowledge of Spanish, French, Italian, or Romanian, many of these terms will be familiar to you. The torqular hirophili (literally, "gutter of Herophilos," a Greek anatomist), for example, is the anatomical term for the confluence of sinuses at the back of the occipital bone, and, as noted above, AB-duction describes a movement of the limb "away from" the body. Other terms are named after the anatomists that discovered or described them, and these often give students difficulty. The torqular hirophili, noted above, is one of these examples. Microscopic lines in the enamel and dentine (two tissues in the teeth) are named striae of Retzius and Owen's lines, respectively. This may at first seem confusing, but don't worry! There is a push in the anatomy community to get rid of terms that have anatomist's names in them, thereby simplifying the study of anatomy. You'll be happy to learn that you are no longer responsible for learning about Wharton's ducts or the mobile wad of Henry —well, at least by those names! — for example. If, for some reason, you are interested in this topic, you can check out the Terminologia Anatomica, which lists the most up-to-date terms for various anatomical structures (you can access it here). The study of anatomy is a fluid, ever-changing topic, and the names of structures change to reflect that.
© 2015 Robert McCarthy