The spinal cord is primarily to be regarded as formed by the union or joining together of many nerve centres, that is to say, of many groups of cells, which commonly act together or in an orderly sequence, producing purposive actions, without any voluntary effort, beyond perhaps supplying the first incentive or stimulus. Even this may be entirely absent, and the animal may have no consciousness of the nerve stimulus or of the muscular actions that follow it. The complete independence of a segment of the spinal cord is well shown in cases where the spinal cord has been crushed in the region of the back by a fall, or a musket-ball, or a sabre-cut. The voluntary movements of the hind limbs are abolished, the animal can neither move them nor feel any injury inflicted upon them. It is said to be paralysed, yet if the skin of the limb be pinched, or touched with a hot body, it will immediately respond by kicking, or by some other spasmodic movement of the paralysed limb. Similarly, by appropriate stimulation the bladder or the rectum may be made to discharge its contents. Such actions or movements are said to be reflex. It can be shown that a stimulus applied to the skin excites a wave which travels up the nerve, enters the cord by the superior root of one or more of the spinal nerves, and reaches one of the nerve-cells in the superior cornu. From this it passes into the inferior cornu and reaches one of the motor cells, and this immediately liberates an impulse, which, emerging by the inferior root, travels down the motor nerve to the muscle. As many sensory nerves are always stimulated, many motor nerves are called into action, and these are so connected and associated together as to produce purposive movements.
In the illustration just given, the reflex movements are said to be without consciousness, and they are analogous to those that are constantly taking place in the uninjured animal in the movements of the intestine, of the heart and blood-vessels, of respiration, and of the ducts of glands; but in a large number of cases reflex acts are accompanied by consciousness, as in the case of winking when the eyelashes are touched or the eye is exposed to a bright light, or of coughing or vomiting from tickling the throat with a feather, or of micturition from over-distension of the bladder. When the animal desires to perform one of these purposive and complex movements, it does not transmit a separate impulse to the several muscles implicated in the act, for it knows nothing of them, but by an act of the mind it transmits a mandate to a group of cells which have learned to act together in a definite order and to produce the required effect. Such centres are named co-ordinating centres or nuclei, and of these there are many, as, for example, those governing the movements of the rectum and bladder in the acts of discharging the faeces and urine; those required for parturition, and for the erection of the penis and the ejaculation of semen, which are chiefly situated in the lumbar and sacral regions of the cord; and finally, the contraction of the blood-vessels of the abdomen and lower limbs, which are chiefly situated in the dorsal region.
There appears also to be present in the cord, centres that control the production of animal heat and of the secretion of sweat, these effects being in part due in both instances to changes in the size of the blood-vessels, and in the case of the sweat secretion to direct action of the nerves on the sweat glands.
We must regard the cord as endowed to an eminent degree with impressionability, and the power of inducing reflex acts without consciousness. But the cord is not a receiver of nervous impressions and a generator of nerve impulses only; it is also a conductor transmitting impressions made upon the skin to the medulla oblongata, cerebellum, and cerebrum on the one hand, and impulses originating in these parts to the muscles of the limbs and trunk, and to the other organs of the body.
The medulla oblongata, while it is a prolongation upwards of the spinal cord, and transmits impressions both from the cord to the brain and cerebellum, and downwards from these parts to the cord, is also itself a very important centre, containing many groups of cells which preside over and govern the complex muscular movements of mastication, insalivation, deglutition, winking, breathing, with its accessory movements of coughing and sneezing.
The corpora striata and optic thalami, or great ganglia, at the base of the brain, are probably the regions where consciousness first appears, consciousness of the different forms and kinds of nervous stimuli, and the place where conscious efforts or muscular movements are made in response to them. These ganglia are particularly connected with the sense of sight. They are relatively large in the horse, for whilst in man their proportion to the brain may be taken as 5: 100, in the horse it is 13:100.
The outer layer or cortex of the brain is the highest centre of all. It is the seat of the emotions of judgment, memory, reason, and the will. We have seen that it consists of gray substance containing many nerve-cells, which give off fibres that extend to and from the ganglia below and to the periphery of the body. It has often been exposed as the result of accident, and has uniformly been found to be insensitive to direct stimulation, so that large portions have been cut away without pain being experienced even in man. Evidence has accumulated during the past few years, showing that the several convolutions have definite functions, so that one set is concerned with the initiation of movements of the head and neck, another with those of the fore-limbs, and others with those of the trunk and hind-limbs. Special lobes of the brain are also connected with the several senses, the occipital lobes being especially connected with the visual sense, the temporo-sphenoidal lobes with the hearing.
The horse appears to be an animal endowed with a remarkable power of association of definite movements with certain mental stimuli, and with an excellent memory. It will stop before the customers' doors, the sound of the rider's voice will cheer and direct it, and in military evolutions the bugle-calls are quite as well known by the horse as by its rider. It will remember events that are long past. It enters into the spirit of trials of speed and strength, and of games, as those of polo and steeple-chasing, with the utmost zest and enjoyment.
The horse owes his proud position with the dog, as the friend of man, to his docility, his gentleness, his great muscular strength and swiftness.