Page:The Galaxy, Volume 6.djvu/55

1868.] into a threatening attitude, and, when irritated, strike its bleeding trunk against the offending body. Upon one occasion, a man had decapitated one of these reptiles, and while bending down to examine it more carefully was struck by it full in the forehead. So powerful was the shock to his nervous system that he fainted, and remained insensible for several minutes. According to Maine de Biran, Perrault reports that a viper whose head had been cut off moved determinately toward its hole in the wall. Analogous movements in man are very effectively carried on during sleep—that condition in which the influence of the brain upon the body is for a time suspended. During this state, if the position in bed be uneasy, it is changed; if the feet be tickled they are drawn away; if the skin be gently pricked, movements are excited, and all these without the slightest agency of the brain, which is not even cognizant of what is going on. They are all set in action by the spinal cord, which never sleeps. And when we reflect that this organ, when acting independently of the brain, is cut off from all communication with the external world by means of the senses of sight, hearing, smell, and taste, and has only that of touch to bring it in relation with impressions and objects, we may be still more surprised at these evidences of its activity.

Such facts certainly seem to show that the nerve centre in question possesses the power of perception, for the movements are such as the occasion giving rise to them ought naturally to produce, and are not those indeterminate actions performed without an object. Such is not, however, the explanation ordinarily given by physiologists, who generally regard them as being examples of what is called reflex action. According to this theory an irritation made upon the extremity of a sensory nerve is conveyed to the spinal cord and is there converted into a motor impulse, which returns to the point of origin through the motor nerve. But this theory, though now almost universally accepted, requires too much to be taken for granted, and must, as science and thought advance, inevitably yield to the one first stated. For (if no other objection be alleged) why should any precise and definite motion be performed unless the spinal cord were aware of the character and extent of the irritation? If the cord is simply a converter of impressions into motor impulses the resultant movements would be altogether without reference to the object to be attained, whereas anyone who observes them as they occur in decapitated frogs will at once perceive how exactly they are in relation with what would normally be the desire of the animal.

But besides originating its own force the spinal cord serves as a conductor for the nerve power which comes from the brain—distributing it to all parts of the body through the several pairs of nerves which it gives oft". Each of these spinal nerves arises by two roots. Of these the anterior is exclusively motor, and the posterior is entirely sensory. Thus if the anterior root be divided and the cut extremity not attached to the skin be irritated, movements ensue in the muscles to which the nerve is distributed, but no pain. Irritation of the other cut end produces no effect whatever. This shows the two facts that the anterior root is the root of motion, and that the motor impulse travels from the spine to the extremities of the nerve. On the other hand, if the posterior root be divided and the unattached end be irritated, no effect is produced, but if the irritation be applied to the extremity in connection with the cord pain is felt. This experiment shows that the posterior root is the root of sensation, and that sensibility travels from the periphery to the centre.

Very soon after being given off, the two roots unite, and thus a spinal nerve,