In considering the functions of the spinal cord, we have to regard it in two distinct points of view—viz., in the first place, as a conductor of nervous force between the nerve-trunks and the brain; and in the second place, as an independent nervous center. As a mere conductor of nervous force, its functions and behavior are the same as those of a nerve-trunk; for, as Dr. Carpenter observes: " If it be divided, all the parts of the body which are solely supplied by nerves coming off below the point of section are com pletely paralyzed, as far as regards sensibility and voluntary movement; no impressions made upon them having the least power to affect the consciousness, and ne exertion of the will being able to determine contraction of the muscles. This state of paraplegia, which may be experimentally induced in animals, is frequently exhibited in man, as a result of injury or of disease that seriously implicates the spinal cord; and as it has been shown that among the lower animals complete reunion of the cord may take place after complete division, as indicated by the entire restoration of its functional powers, and the complete redintegration of its structure, so have we reason to believe that a similar regeneration may take place, to a considerable extent, in man, this being marked by a gradual return of sensibility and power of voluntary movement in the lower limbs, which had been at first completely paralyzed."—lluillan Physiology, 6th ed., pp. 529-30. There can be little doubt that the gray matter is essentially the conductor of sensory impressions, for if the anterior, posterior, and antero-lateral columns are divided as com pletely as possible, the gray substance remaining uninjured, the sensibility of the parts below is unaffected; while, conversely, if the gray substance is divided, while the white columns remain uninjured, sensibility is almost totally extinguished. M. Brown-Sequard, whose researches on the nervous system are of the highest impor tance, has shown that the central portions of the gray substance are the most effective in the transmission of sensation. Ile likewise brings forward strong evidence to prove that there are special conductors in the spinal cord for the sensations of touch, pain, temperature, and muscular contraction, none of which can convey other sensations than their own. Notwithstanding its singular power of sensory impressions, the gray substance is itself insensible. Among his other remarkable discoveries in connection with this subject, Brown-Sequard has found, that on dividing one-half of the spinal cord of an animal, not only is anaesthesia (or loss of sensation) established on the opposite side of the body, but there is also produced a state of hypersesthesia (or exalted sensibility) on the same side, which begins to appear a few hours after the operation, and continues in dogs for about 20 days, in cats about 14 days, and in guinea-pigs for many months, after which the sensibility falls below its usual standard. With regard to the conduction of motor impulses, there is great uncer
tainty. Considerable differences have been shown to exist in the position of the motor tracts in different parts of the cord, and Brown-Sequard concludes from his experhnen s on the effects of section, that while in the dorsal region, all parts, except the postermr columns, are employed in the conveyance of the orders of the will to the muscles, in the upper part of the cervical region, most of these conductors are in the lateral columns and in the gray substance between these and the anterior column.
We have now to consider the spinal cord as an independent nervous center. The sim plest, and, at the same time, the most decisive evidence of the independent power of the spinal cord, is derived from the motion exhibited by the limbs of animals when irrita tion is applied to them after section of the cord at some point above the entrance of their nerves; the fact that these movements are reflected through the cord, and do not result from direct stimulation of the part irritated, being shown by their complete cessation when the nerve-trunks are divided. Thus, if a frog be piffled by dividing the cord between the occipital foramen and the first vertebra, an unusual convulsion takes place while the knife passes through the nervous center; but this quickly subsides, and if the animal be placed on the table it will resume its ordinary position. It is quite unable to move by any voluntary effort; but if a toe be pinched, the limb is instantly drawn up, and seen tO push away the irritating agent, and then draw up the leg again to the old position.
From these and other experiments, we may conclude (1) that the spinal cord in union with the brain, is the instrument of sensation and voluntary motion to the trunk and extremities; and (2) that the spinal cord may be the medium for the excitation of move ments, independently of volition or sensation, either by direct irritation of its substance, or by the influence of a stimulus conveyed to it from some surface of the trunk or extremities by its nerves distributed upon that surface.—For further information on this subject, the reader is referred to Todd and Bowman's Physiological Anatomy and Physi ology of _Alan, 2d ed., vol. i.; Carpenter's human Physiology, 8th ed. 1876; and the °tint standard works on physiology.