Like those which spring from the spinal cord, the nerves of the brain are divisible into motor and sensory nerves. Among these last some are endowed with a special sensibility, as the olfactory, the optic, and auditory nerves; the others transmit general sensations. Several of the cranial nerves are made up of filaments of different orders, and are formed by the union of nerves of general sensation, of special sensation, or of motion. Like the spinal nerves after the union of their roots, they form cords, mixed in their functions as a whole, but distinct in those of their several filaments. The analogy between the cranial and the spinal nerves is completed by the branches which go from the cranial nerves of sensation to the great sympathetic, and by the gray fibres which are seen near the origin of the cranial nerves, and also near the posterior roots of the spinal nerves. In the cranium the motor nerves emerge from the prolongation of the anterior fascicles of the cord in which the spinal motor nerves originate.
We have already seen that the anterior fascicles of the spinal cord are insensitive, and that they give motor power to the anterior roots of the nerve, while the posterior portions are sensitive, like the nerves which emerge from them. These properties of the medullary fascicles were for a long period disputed, but they have been clearly demonstrated by M. Longet's experiments. The spinal cord imparts to the nerves of the trunk and limbs the power of voluntary and respiratory motion. It is also the source of nervous energy in the action of the heart, and in the circulation of the blood, in the phenomena of nutrition and of secretion; lastly, it seems to have only an indirect influence on the production and maintenance of animal heat. When there is any lesion of one of the lateral halves of the cord, it is on the corresponding side of the body that motion and sensation are disturbed or destroyed. The action therefore of the spinal cord is direct on the organs to which it sends nerves, and not crossed like that of the encephalon.