Figures of speech that employ characteristics associated with a horse's head are not always terms of endearment. To say that a person has a horse face, horse teeth, or a horse laugh or to suggest that he lacks horse sense or that he eats like a horse is to risk riding roughshod over his psyche. Yet these third-person-derisive applications are thumpingly ironic, for the equine head is the crown jewel of a crowning achievement: a peerless and complex creation that mimics and abets the various systems of the body that supports it.
We can appreciate the significance of the head among the world's fauna when we consider the multitude of non-living things that are said to possess one. Heads are standard equipment on pins, nails, tennis rackets, and plants, on rocks, rivers, tools, coins, beers, drums, and fiddles, too.
Nor has human action been spared the head's metaphorical embrace. Athletes sometimes play over their heads; movie stars are known as headliners; and when we are overly impressed, our heads get turned--by a comely head on a horse, no doubt. For when it comes to nobility of bearing, the head of a horse is at the head of its class.
The Brain It Is Plain
The star pupil in the horse's head is the brain, the 5 " by 4" valedictorian of the central nervous system. Headquartered in the cranial cavity, the brain weighs about 24 ounces and comprises 1/7 of 1 percent of the weight of a 1,000-pound horse. (The brain of a 1,000-pound cow comprises 1/10 of 1 percent of its weight; and the human brain, which weighs 48 ounces, comprises 1.7 percent of the weight of the average 5' 10" male between the ages of 30 and 39.) An enlarged and highly modified continuation of the spinal chord, the brain is divided into two, ovoid-shaped hemispheres whose surfaces are marked by numerous, thick folds. These cerebral hemispheres make up the greater part of the fully developed brain.
The brain is lovingly embraced by the snug-fitting cranial cavity. The posterior wall of this protective compartment (and a portion of its base as well) is formed by the occipital bone, whose lower section is perforated centrally by an almost circular opening--at which opportunity the spinal chord blossoms into cephalic flower. The anterior wall of the cranial cavity is fashioned from a plate of the ethmoid bone, which separates the cranial and the nasal vaults. Various permutations of the occipital, supraoccipital, parietal, interparietal, temporal, frontal, and sphenoid bones form the roof, floor, and lateral walls of the brain's pearly apartment.
Like wires running to the dashboard of a mighty 18-wheeler, 12 pair of cranial nerves keep the horse's brain informed of developments affecting the sensory and motor systems. Three of these 12 pair of nerves--the optic, olfactory, and auditory--serve as couriers of the special senses: sight, sound, and smell. Five pair of cranial nerves--the oculomotor, abducent, hypoglossal, trochlear, and spinal accessory--pledge their allegiance to the motor functions; and the remaining four pair--the trigeminal, facial, glosso-pharyngeal, and vagus--straddle both sides of the sensory-motor highway.
Ahead of Its Time
The head--called by Lord Byron "the dome of Thought, the palace of the Soul"--began to acquire primacy as evolution proceeded in the aftermath of the Big Bang. Gradually, the one-celled organisms that were the earliest manifestations of life banded together to form colonies. When the size of these colonies made the acquisition of oxygen and nutrients and the elimination of waste difficult, division of labor was invented. Ad hoc assemblies formed within colonies to satisfy the vital functions of life, including ingestion, digestion, excretion, reproduction, and--in mammals--temperature control.
As cell colonies progressively redefined themselves into sponges, tubeworms, crustaceans, mollusks, fishes, and mammals, the head of each evolving organism--besides playing host to the brain--reserved more and more of the action vis-a-vis the body's functions. Consequently, air intake, food intake, and excretory capabilities can all be found in the horse's head, which acts, in a sense, as the body in microcosm. The head, for example, produces some digestive enzymes in the salivary glands, located on either side of the face and the adjacent parts of the neck. The largest of these glands is the parotid, named for its proximity to the horse's ear. This gland is eight to ten inches long, nearly an inch thick, and weighs about seven ounces--more than 1/4 as much as the horse's brain.
Among the substances absorbed in the head are essential oils, which contribute much to the flavor of life. You can feel an example of this absorption when you suck on a cough drop and the tingling in your face tells you that your mucosa is taking up the camphor in the drop. In similar manner, metallic salts and other relatively uncomplicated materials, including the rabies virus, are shed in saliva.
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