THE CENTRAL, OR CEREBROSPINAL, NERVOUS SYSTEM.
It has been said that however complicated a nervous structure may appear to be it consists essentially of nerve-cells and nerve-fibres.
In the lower animals, the invertebrates, or those which have no backbone, cells are situ ated in groups in certain places in the body, and the fibres radiate from them to the various parts of the body, the groups being somewhat loosely connected with one another. But in vertebrates, animals having a backbone, the groups of cells are more closely connected to gether, and form central masses from which large trunks of nerves issue to be distributed to the various parts of the body. Thus one speaks of the central nervous system. In the higher animals and man there are two such central masses, which for better protection are surrounded by bone. One such mass, of a more or less globular form, is situated in the upper or forward end of the animal, inclosed within the skull, forming the cerebral mass or brain or encephalon, and the other, of an elongated columnar form, is situated within the central canal of the spinal column or back bone, forming the spinal mass or column, or spinal marrow, and these two are so connected that the spinal marrow becomes a prolongation of the brain mass. For this reason this central nervous system is also called the cerebrospinal nervous system.
Before discussing the relations between these structures and the duties they perform, we shall describe their appearance and general arrange ments. A general view of their position is shown in Fig. 83.
The brain proper consists of the cerebrum, or larger brain, which occupies the whole of the upper and front parts of the cavity of the skull, the cerebellum, or lesser brain, lying beneath the hinder part of the cerebrum, and the medulla oblongata, or oblong marrow, which may be regarded as a continuation of the spinal cord within the cavity of the skull, and as form ing the connection between the brain and cord. Included also in the brain are certain masses of nervous matter to be afterwards described, lying towards the floor of the cavity of the skull, covered over and concealed by the larger brain, and called basal gan glia.
The cerebrum and cerebellum are almost com pletely divided into two lateral halves by a deep longitudinal fis sure, and the surface of the former is divided by a considerable number of ir regular furrows, nearly an inch deep, into convo lutions. As the gray matter of the brain spread out on its surface is the portion having the highest func tions, its quantity is largely in creased by being thus thrown into convolutions.
The Cerebrum is oval in form, arched above and somewhat tened on its lower surface, which rests on the floor of the skull. Usually its an terior or frontal portion is somewhat nar rower than the hinder portion, and its greatest breadth is between the ears. The great fissure, running from before backwards, divides it into two hemispheres, but these are connected by a large bawl of nervous matter, seen in Fig. 86, B, called the corpus callosum. Each hemi sphere is divided by anatomists into anterior, middle, and posterior lobes, corresponding generally to the same regions of the skull. The general appearance of the surface of the cere brum is seen in Fig. 84.
The under surface of the brain (Fig. 85), which rests on the floor of the skull, shows the origins of the important nerves, called the cranial nerves, time cerebellum, the optic commissure (2-2) or structure connecting the optic nerves, the pons Varolii (rv) or bridge of nervous matter connecting together the two hemispheres of the cerebellum, and lastly numer ous convolutions.
Basal we divide time brain into two Iiortions by cutting lengthwise through the great longitudinal fissure, and dissecting a short way towards either side, we find that each hemisphere covers over several large masses of nervous matter, which have been called the ganglia at the base of the brain. These are, from before backwards: (1) Two bodies streaked on the surface, and hence called corpora striata, or stri ated bodies; (2) Two bodies behind and a little to the outer side of the corpora striata, sup posed by the older anatomists to be connected with vision, and hence called the optic thalami, or optic masses; and (3) Four bodies, two on each side, called corpora quadrigemina, or bodies.