When discussing anatomy of any animal, it's important to first gain a point of reference and establish terminology for description. We can't use terms like "over" or "under" or "in front" or "behind." It's fine to say that the trachea is in front of the esophagus when we are looking at a penguin from the front, but the esophagus is in front of the trachea if we look at the penguin from behind. Vessels can run under a tissue from one point of view or over a tissue from another point of view. Therefore, to be consistent, we establish terms that apply to location from any point of view. Look at the following drawings to learn the anatomical planes.

    The Adelie penguin on the left of the screen is bisected by three planes. You can see a plane running parallel to the ground, running right through the abdomen of the penguin. This is called the horizontal or transverse plane. There isn't just one horizontal plane; there can be many horizontal planes. However, all horizontal planes are parallel to the ground. The horizontal plane divides the penguin into top and bottom portions. I'll go into more specific terms for "top" and "bottom" a little later. This is called the axial plane as well

    The next plane which is visible appears as a rectangle running around the penguin. This plane runs perpendicular to the ground and divides the penguin into front and back portions. This is called the frontal or coronal plane. Like the horizontal plane, there can be many coronal planes dividing up the penguin.

    The last plane appears as a thin line running through the center of the penguin. This is called the median plane or sagittal. There is only one median plane and only one sagittal plane since by definition the "median" is the absolute middle. The sagittal plane is named after the mythological creature Sagittarius whose arrow would pierce victims and split them directly into two equal halves. Planes bisecting the penguin to the left or right of the median plane are called parasagittal planes.

    Why are these planes useful? Well, they are useful when we talk about parts of the penguin. For example, a vessel may run in the median of the penguin. When I say that, you might imagine that the vessel runs directly down the middle of the penguin. What if I say the nerve runs horizontally under the rib? You might imagine that this nerve runs in the horizontal plane. So, the purpose of planes is to give a frame of reference.

    Planes are only so useful. The terms that are more useful are the directional terms that indicate the position of organs or tissues. Look that the image below for some basic anatomical terms.

    First let's talk about the terms dorsal and ventral. Dorsal is what we think of when we think of the back of the penguin. However, we can't say that the erector muscles of the back are "back to" the spinal column. But, we can say that the erector muscles are dorsal to the spinal column. (We can also say "superficial" or "posterior" to in this case.) So, dorsal refers to the back. If dorsal refers to the back, then ventral must refer to the front or "belly" of the penguin. So, if I say the nerve runs along the ventral aspect of the penguin, you might imagine that the nerve runs along the "belly-side" of the penguin.

    The second terms we use are cranial and caudal. Cranial means "towards the brain or cranium." Caudal means "towards the tail." Cranial is sometimes used interchangeably with superior, and caudal is sometimes used interchangeably with inferior. So, if I say that the foot is caudal to the knee, you know that the foot is anatomically "below" the knee. This is a ridiculous example, but you get the idea.

    The last three terms are a little more confusing. First, the term rostral means "towards the nose." This doesn't seem too confusing, but sometimes rostral is used to mean "towards the head," which you know is "cranial." To understand which they mean one has to read closely. The last terms to know are proximal and distal. They are used to clarify "origin" and "termination." So, a structure that is proximal to another structure is a structure that is closer to the origin. Look at the flipper in the drawing. The "axilla" is the "armpit" of the penguin. Would you say that the axilla is proximal or distal to the tip of the flipper? Although it is far away from the tip of the flipper, anatomically, it is proximal to the tip of the flipper. "Proximal" and "distal" can be confusing terms, but all you need is a little practice.

    So let's dive right into penguin anatomy, and we'll start with penguin embryology. Penguins start somewhere, you know...