Two substances, distinguishable by their color, namely, the white or medullary and the gray or cortical substance, enter into the formation of nervous matter. Both are soft, fragile, and easily injured, in consequence of which the principal nervous centers are well protected by bony coverings. The nervous substances present two distinct forms--nerve fibers and nerve cells. An aggregation of nerve cells constitutes a nerve ganglion.
The nerve fibers represent a conducting apparatus and serve to place the central nervous organs in connection with peripheral end organs. The nerve cells, however, besides transmitting impulses, act as physiological centers for automatic, or reflex, movements, and also for the sensory, perceptive, trophic, and secretory functions. A nerve consists of a bundle of tubular fibers, held together by a dense areolar tissue, and inclosed in a membranous sheath--the neurilemma. Nerve fibers possess no elasticity, but are very strong. Divided nerves do not retract.
Nerves are thrown into a state of excitement when stimulated, and are, therefore, said to possess excitable or irritable properties. The stimuli may be applied to, or may act upon, any part of the nerve. Nerves may be paralyzed by continuous pressure being applied. When the nerves divide into branches, there is never any splitting up of their ultimate fibers, nor yet is there ever any coalescing of them; they retain their individuality from their source to their termination.
Nerves which convey impressions to the centers are termed sensory, or centripetal, and those which transmit stimulus from the centers to organs of motion are termed motor, or centrifugal. The function of the nervous system may, therefore, be defined in the simplest terms, as follows: It is intended to associate the different parts of the body in such a manner that stimulus applied to one organ may excite or depress the activity of another.
The brain is that portion of the cerebrospinal axis within the cranium, which may be divided into four parts--the medulla oblongata, the cerebellum, the pons Varolii, and the cerebrum--and it is covered by three membranes, called the meninges. The outer of these membranes, the dura mater, is a thick, white, fibrous membrane which lines the cavity of the cranium, forming the internal periosteum of the bones; it is continuous with the spinal cord to the extremity of the canal. The second, the arachnoid, is a delicate serous membrane, and loosely envelops the brain and spinal cord; it forms two layers, having between them the arachnoid space which contains the cerebrospinal fluid, the use of which is to protect the spinal cord and brain from pressure. The third, or inner, the pia mater, is closely adherent to the entire surface of the brain, but is much thinner and more vascular than when it reaches the spinal cord, which it also envelops, and is continued to form the sheaths of the spinal nerves.