...Connected to the Backbone

by James R. Rooney

The centerfold in the Holiday 1998 issue of Rural Heritage shows the articulated skeleton of a horse. Since a full consideration of how the bony skeleton is put together would require a lot of words and a lot of pages, let's look at only a limited aspect of the functional anatomy of a horse's skeleton.

Both "function" and "anatomy" are perfectly clear words when each is taken by itself. "Anatomy" comes from ancient Greek and means to dissect or take apart. "Function" is a Latin word meaning to perform or carry out some act. Let's do some dissection of the horse's bone structure to determine how the parts we take apart work in life. "Skeleton" is another word of Greek origin meaning withered or dried up. The word refers to the hard parts of the horse's anatomy.

The skeleton is composed of bones. That word (yes, I am going to do it again) is derived from old and middle English and old Norse languages, and refers to the parts of the skeleton as we know them. Bones form the framework, the basic armature upon which and within which the soft tissues of the body attach or reside. Without bones and the skeleton made up of bones an animal would be shapeless, simply conforming to the surface upon which it is pressed by gravity—something like an amoeba, or gelatin gradually spreading on a plate.

The most elemental function of the skeleton is to provide the stiff, rigid part of the mechanism for resisting the force of gravity. While many other forces play upon the horse's body throughout its lifetime, the one constant force, always present, is the gravitational force. Today let's concern ourselves with the backbone—the vertebral column—and some aspects of the basic functional anatomy (in short, the mechanics) of that structure.

The vertebral column is composed of a series of vertebra joined together by ligaments and disks, giving the vertebral column per se some degree of flexibility.

Let's use a routine trick of mechanics—simplifying first in order to develop a basic understanding of how something works. With such understanding we can return to the complex with greater ability to understand the details of the complex structure. Accordingly, as a drastically simplified representation of the horse's back, consider the vertebral column to be a beam supported at one end by a column and at a distance from the other end by another column. The overhanging part of the beam represents the head and neck.

This illustration indicates another mechanical trick. The entire mass of the horse is considered to be concentrated at a single point called the center of mass, also called the center of gravity or the center of balance or equilibrium. If we suspended a horse (or a model of a horse) from the ceiling by a rope, it would be balanced when the rope was attached at or very near the center of mass. The purpose of using a center of mass is to simplify understanding the mechanics of the horse.

We know that gravity always acts towards the center of the earth, toward the surface, thus the arrow pointing downward at the center of balance. It is apparent that this gravitational force will tend to bend the back of the horse downward. It is also apparent that the back does not normally bend like that. Why not?

The back does not bend because it has ligaments, tendons, and muscles that lift the back up, counteracting the effect of gravity. Because of the way those soft tissues are arranged, however, their action not only lifts the back but also stiffens it. There's good reason for that. The vertebral column is a series of vertebrae attached to each other, with movement possible between any two vertebrae and, so, movement of the whole column. You can appreciate this in the slowly walking, live horse by walking along behind and watching the sinuous, snake-like movement of the back from side to side. If this isn't too clear, draw a chalk line down the middle of the back and watch it as the horse moves.

Now move the horse along faster up to a trot, and you'll see that the snaky movement decreases—the back becomes stiffer and stiffer. This same increase in stiffness occurs if the horse continues at the same slow walk on a draft load that is gradually increased.

The back stiffens in this way because the muscles of the back are contracting more and more in order to balance the increasing force being delivered to the back (indeed to the whole horse) by the legs in order to achieve greater speed and/or to move a heavier draft load.

So, why not just make the back very stiff to begin with rather than flexible? Well, a horse needs to have some flexibility of the back in order to lie down and get up, graze, bite at flies and people, mount mares, and so on. The wonder of this system is that the horse can do all these things requiring flexibility and still be able to stiffen the back in order to balance the propulsion forces of its legs. If the "backbone" were, indeed, a simple piece of bone or a beam, it could be stiff enough, but would not have the flexibility provided by the series of vertebrae joined together by ligaments and disks and stiffened or loosened by the muscles of the back.

So far we have considered only part of the vertebral column. What about the neck? Let's add the effect of gravity on the head and neck.

Just as before we need a counterbalance to this gravity effect to keep the horse's nose off the ground. The basic counterbalance is the strong, elastic band, the ligamentum nuchae (which has no common name, so I'll refer to it as l. nuchae.) This elastic ligament is much like a large rubber band that provides passive resistance to gravity, acting in the same manner as a strut supporting a shop sign.

With no muscular work, then, the head and neck are held in a normal resting position by the l. nuchae. If the horse wishes to graze, muscles overcome the elastic resistance of the l. nuchae, pulling the head down to the grass. If he wishes to throw his head up to see what's going on down the field, the muscles pull up. The l. nuchae, then, is a nice energy-saving device for supporting about 10% of the horse's weight (the head and neck), at the same time permitting the muscles to move the head and neck around for various purposes.

The general result of all this is that the vertebral column can be flexible or stiff, move or rest, as needed and with minimal expenditure of expensive energy. This general rule applies to virtually all the bones and joints in the horse's body. The joints are moveable in order to allow for movement over the ground and can be stiffened—stopped from moving—in order to stop movement and keep the horse upright against the ubiquitous force of gravity.

James R. Rooney, DVM, is retired as a professor of veterinary pathology at the University of Kentucky. His relationship with horses began when he was a boy working draft horses on a farm in Southern Maryland. Today he studies the functional anatomy and pathology of horses of all breeds. He is author of The Lame Horse and two companion videos: Guide to Lameness—Front Legs and Guide to Lameness—Hind Legs and Back. This article appeared in the Holiday 1998 issue of Rural Heritage.