A FEW WORDS ABOUT THE NERVES.
I.
THE BRAIN AND SPINAL CORD.
IF there is any one feature in the organization of man which makes him superior to all other created beings, it is the extent and delicacy of his nervous system. Some animals may excel him in acuteness of sight, of hearing, or of smell, but they employ these senses almost entirely as means of securing their food or ensuring their safety from enemies, while man makes them minister in a thousand ways to his physical or intellectual enjoyment. As regards touch and taste, in no animal do they reach the high degree of refinement to which they are developed in man.
The nervous system in man consists of the brain, the spinal cord, the sympathetic ganglia, and numerous nerves. Each of these exists in a more or less highly elaborated form in all vertebrates. As we descend in the scale we find first one and then another becoming less perfect, or altogether disappearing until we reach those dubious beings scarcely animal, though more than vegetable, in which no traces of a nervous system have yet been discovered.
Two very different kinds of tissue enter into the composition of the nervous substance. One of these, the white, is formed of fibres, or rather tubes. It serves merely for the transmission of impressions and the mandates of the will, and has nothing to do with the origination of thoughts or nervous force. The nerves which are distributed to the different parts of the body are composed entirely of this white matter. It is likewise found in large amount in the brain and spinal cord.
The other, the gray substance, is situated upon the outside of the brain where it forms a thin coating, and in small isolated masses placed at its base. It is also present in the centre of the spinal cord throughout its length, and in certain enlargements which exist on the posterior roots of the spinal nerves and along the course of the sympathetic nerve. A mass of gray matter is called a ganglion or nerve-centre. The brain consists of several ganglia—the cerebrum, the cerebellum, the optic thalami, and others. The spinal cord is also a ganglion, and the sympathetic nerve has many which are in direct relation with the heart, the stomach, and other organs. The gray matter originates ideas, emotions, the will, and all nervous force of whatever kind. It is the perceptive part of the nervous system.
In the higher members of the class mammalia the surface of the brain is not smooth as it is in the lower animals, but is marked by convolutions with fissures between them. Among the animals thus characterized are those which are most remarkable for their intelligence, such as man, the monkeys, the dog, the bear, the seal, the elephant, and the horse, while among those with smooth-surfaced
