Page:Popular Science Monthly Volume 26.djvu/759

Rh by nerve-matter, and by the special adjustments of this matter found in the spinal cord. It is interesting to remember, in this connection, that the motions which may be the occasion of sensation are carried to the posterior roots of the cord, while those motions which result in movements are carried to the anterior roots of this organ.

These roots are the crescentic-shaped arrangements of cell-substance before described. The functions of the cord are not limited to transmission. The cord is the source of independent or reflex activities. The peculiarity of these activities is that no consciousness and no volition accompany or occasion them; they are strictly motions. In swallowing food we have an illustration of these reflex activities, and of their close succession to activities that were both conscious and voluntary. Consciously and voluntarily the food is carried to the fauces; at once, the excitation made by the food upon the afferent nerves is carried to the cord and the medulla oblongata; here force is liberated and sent along efferent nerves to the muscular walls of the œsophagus. These walls contract, and the food is passed on into the stomach.

When the cord is broken, those parts of the body which lie below the break will move violently upon irritation, though they can not be moved by any effort of will or be known by any sensation. Many actions, not at first reflex, become so by repetition. Walking is a good example: the movements are learned slowly, and upon numerous efforts; afterward, the work is performed by the centers of the spinal cord, so that walking is really hindered by conscious volition. Dr. Carpenter mentions the case of a shoemaker who was subject to sudden loss of consciousness; at such times he always continued the work he was engaged in when consciousness left him; if walking, he would walk into water or fire; if using his awl, he would continue doing so, frequently to his serious injury.

While this reflex action of the cord may thus take place apart from the brain, the brain exercises a strong inhibitory influence over the action. Some persons, by sheer force of will, can hold their feet still under constant tickling of the soles. The following experiment seems