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Remote Control
In addition to directing sensory and motor activities and to participating in the vital functions of the body at large, the head, by virtue of the title it holds to the pituitary gland--the principal control gland of the body--wields clout with the endocrine system as well. Nearly a dozen functions in the body--from growth to the onset of estrus to the amount of urine secreted by the kidneys--are controlled by hormones manufactured, released, or stored by the anterior and posterior lobes of the pituitary, a yellowish-brown, discoid structure connected to the base of the brain by a hollow stalk.
Because of its evolutionary history, its custody of the brain, its stewardship of the pituitary gland, and its top-secret access to feedback from all precincts of the body via the nervous and circulatory systems, the head is the MVP--most valuable part
--of the horse. Indeed, the horse's body can be thought of as a member of the servant class, dedicated to obliging the needs and dictates of its headstrong master. The muscles of the legs and back, to cite an example, exist to transport the head so that it can acquire food and participate in the other functions essential to life. (If you've got any problem with that, try to remember the last time you heard about somebody being paralyzed from the neck up and still being able to maintain numerous corporal functions.)
Such being the luck of the draw, it is not surprising that the head should claim absolute priority to the body's oxygen and nutrient supplies and to its circulatory capacity. (Although it comprises less than 10 percent of the horse's total weight, the head commands 25 to 30 percent of the body's circulation.) Accordingly, should the head be in extremis, the horse's regulatory mechanism will direct other parts of the body to concede their privileges and to allow the commitment of necessary resources to the maintenance of the head--specifically, of the brain. This occurs, as that old equestrian Cervantes once remarked, because "when the head aches, all the members partake of the pains."
The circulatory system, in fact, has nervous control mechanisms which deprive the guts, the legs, and the feet of blood in order to make sure that the horse's head gets an adequate supply. What's more, the brain lives on a diet of simple chemicals. Other cells throughout the body have to work harder for their nutrients--and they have the capacity to do so, but the brain lives well on simple fare.
By Design
The conformation of a horse's head--much longer than it is wide--was determined by the animal's need to forage in order to survive. Foraging influences the extent, shape, and arrangement of a horse's teeth, and chewing determines the distribution of muscles and the shape of the bones in the head. Thus, if it is disfigured by injury, the horse's head will gradually re-assume its normal, elongated shape as the forces which chewing imposes on the bones of the head slowly rearrange its contours.
There is, furthermore, an interesting correlation between the size of a horse's head and the mechanics of his gait. The head constitutes a resistance placed at the extremity of the arm of a lever formed by the neck. Because of the movements the head executes, its size--relative to that of the neck--exerts a great influence on a horse's center of gravity. As a result, the smallest heads are often found on the longest necks, and the biggest heads are frequently seen on the shortest ones. This is so because the sum of the mass of the head and neck, and the distance from the center of the mass of the head and neck to the center of the mass of the body determine the mechanics of balance and the use of the head and neck in motion. A big-headed-short-necked horse has the same front-end mechanics as a horse with a smaller head and a long, narrow neck. (The latter has long been the preferred combination because it is capable of providing greater refinements of action, but the emergence of European warmbloods at the top of performance ranks has restored some of the popularity of thicker, plainer, less elegant heads.)
The placement of the horse's eyes and ears--like the size and the shape of his head--have also been determined with a nod to survival. The eyes are situated so that a horse can see above his feed. A horse grazing in long grass would sooner eat the tops of the grass than lower his head to the point where he can't keep an eye on his surroundings. Ear placement, too, is on the high side of the forage line so that if a horse does have to lower his eyes, he can still rely on his ears for information about his environment.
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