Anatomical body planes

In the last post we covered the anatomical terms for directions. Now we'll discuss planes - not the flying kind, but planes that divide the body into two. Planes can be important when considering medical imaging, embryology (how an embryo develops) and descriptions of body motion.

Please note that different terminology seems to be used about humans and about non-human animals. Here I focus on the terminology used about animals.

Median plane: Median plane divides the body into the right and left half, exactly from the center. 

Sagittal plane: If one divides the body just off the median plane, that would be called a sagittal plane. It still divides the body into right and left, but not from the center. The halves are not identical anymore.

Dorsal plane: Dorsal plane divides the body to dorsal and ventral areas, cutting lenghtwise through the midsection of the body.

Transverse plane: Transverse plane is a local plane, meaning that it's in the right angle to the axis of the thing we're looking at. If you're looking at a leg, the transverse plane would go from the front of the leg to the back, at the right angle to the leg itself. If you're looking at the body, the transverse plane is from the back to the stomach (at a right angle to the spine).

The planes are used when talking about the entire animal. When talking about a skeleton, two more distinctions can be made. Now we'll have axial skeleton, which comprises of the head, the vertebral column (spine) and the ribcage. Ventrally to the axial skeleton is the appendicular skeleton, comprising of the limbs.

Anatomically correct directions

An old Internet story claims that a 10-year old boy described a cow thus:

"A cow has seven sides: upside, downside, upper side, below side, right side, left side and inside. The head is on the upper side."

He is right, in a way, but the description could be more specific. We could say that the cow has head, body and legs.. but what about the neck? Is that a separate part, or does it belong to the head or the body? 

Also, a cow is easy since we all know how they look like. It's relatively easy to say where the head is (in the front, attached to the neck) or where the hind legs are (under the pelvic area, attached to the body). Alright, but where is the liver in relation to the spleen?  Where's the lump you felt, the one kinda close to the ribs but more to the top, if you look directly at the cow and you're about the same height as the cow? There's a wound in the intestines - where is it? 1".5 meters from the start"?

We need anatomical directions. Words that are unambiguous and describe things in relation to the animal itself. With these words we can describe precisely where something is.

Let's start with the main regions. We can divide the body to regions and cavities (hollow areas). A vertebrate has usually

• A head, with three cavities: 
• nasal cavity or rhinarium (the hollow space in the nose)
• oral cavity (mouth)
• the orbit (which contains the eyeballs)
• The neck, attached to the head
• The trunk, with three cavities: 
• the thorax or thoracic cavity (lung area)
• abdomen (stomach(s) and intestines)
• pelvis (urinary tract and reproductive organs)
• Forelegs
• Hind legs

To get more into detail several specific words are used to describe location. Note that they are not related to the animal's position. Think of the phrase "above the dog's hind legs". If the dog is standing, above the legs is the pelvis. If the dog is sitting, above the legs is his stomach!   

Caudal - Cranial

Caudal means to the direction of the tail (or rump in animals without a tail), and cranial means to the direction of the head. For example, an important blood vessel called caudal vena cava , which goes from the heart to the direction of the tail. They can also be used in the meaning "related to or located in", like cranial nerve are nerves emerging from the brain (cranial region). 

Dorsal - Ventral

Dorsal is to the direction of the back, and ventral to the direction of the stomach. Easy to remember when you think about people who "speak from their stomach" - ventriloquists! 

Distal - Proximal

Distal means moving away from the trunk. This could be to any direction, as long as it's away from the trunk. Proximal is to the direction of the trunk. The hooves of an animal are distal to their body, and the shoulders are proximal to the hooves. 

These words have no meaning on some organisms, such as starfish. For these we could use "peripheral" instead of distal - something away from the center.

Medial - Lateral

Let's again use a hind leg as the example. How do we describe something which is on one side of the leg? Right/left is difficult. Are you facing the animal, is it the animal's right/left, and and what if the animal is on it's side? 

Medial refers to "closer to the midline", i.e. inside of the leg. Lateral is away from the midline, i.e. the outside of the leg. Note that the when you face the animal, the right side of the right foreleg is lateral, but the right side of the left foreleg is medial... Hence, medial and lateral rather than right and left.

With these terms we can now say that the lump is 5 cm caudal and 10 cm proximal to the last rib. Or that the horse has soreness on the left medial forelimb, distal to the knee. Much better than "let me show you, if you squeeze here then OOW it kicked me!"

Sources:

Wikipedia https://en.wikipedia.org/wiki/Anatomical_terms_of_location

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