NERVOUS SYSTEM, THE, is composed in all vertebrated animals of two distinct por tions or the and sympathetic or ganglionic.
The cerebrospinal system includes the brain and spinal cord (which form the cerebro spinal axis), and the cranial and spinal nerves. It was termed by Biehat the nervous sys tem of animal life, and comprises all the nervous organs concerned in sensation, volition, and mental action.
The sympathetic system consists essentially of a chain of ganglia connected by nervous cords, extending from the cranium to the pelvis, along each side of the vertebral column, and from which nerves with large ganglionic masses proceed to the viscera and blood vessels in the cavities of the chest, abdomen, and pelvis. It was termed by Bichat the nervous system of_organic life, since it seems to or quite independently of the will—the due performance of the functions of the Organs of- respiration, circula tion, and digestion.
The essential parts of the eerebro-spinal axis are described in the articles BRAIN, CERE I3RUM and CEREBELLUM, and SPINAL Coln). The brain and spinal cord are covered and protected by three membranes or meninges, as they are frequently termed—viz., diet/ter: seater, the arachnoid, and the pie mater. The dam mater is a strong fibrous membrane, which supplies the cranial bones with blood in early life, and adheres tirmlv to their inner surface. It is less closely attached to the bony walls of the spinal canal. Inside the cranium it gives off processes (such as the flax cerebri, tentoriam, cerebelli, and fitlx cerebella) which divide and support different parts of the brain; it gives a strong fibrous sheath to every nerve; and by splitting into two layers at certain points, it forms recep tacles for venous blood, which arc termed SINUSES (1 v.), The arachnoiti(so called from its being supposed to be as thin as a spider's web) is a serous membrane, and, like all serous membranes, is a closed sac, consisting of a parietal and visceral layer. The parietal layer adheres to the inner surface of the dam mater, to which it gives a smooth, polished appearance; while the visceral layer somewhat loosely invests the brain and spinal cord, from direct contact with which, however, it is separated by the intervention of the pia mater and some loose areolar tissue. In most regions there is an interval
between the visceral layer of the arachnoid and the pia ntter, which is called the sub arachnoid cavity, and is filled during life by the cerebrospinal fluid. This fluid, winch varies in quantity from two to ten ounces, keeps the opposed surfaces of the arachnoid in close contact, and affords mechanical proteetibn to the nervous centers which it sur rounds, and guards them against external shocks. It is accumulated in considerable quantity at the base of the brain, Where it serves for the protection of the large vessels and nerves situated there. In fracture of the base of the skull, the draining away of this fluid, often in very large quantity, through the external auditory meatus, is often one of the most significant symptoms. It is doubtless secreted by the pia mater, which is the immediate investing membrane of the brain and spinal cord. This membrane consists of minute blood-vessels, held together by an extremely fine areolar tissue. It dips down between the convolutions and fissures of the brain, and is prolonged into the interior, forming the velum, interpositam and the choroid plexuses of the fourth ventricle. It is by means of this membrane that the blood-vessels are conveyed into the the nervous substance.
We now proceed to notice the nerves connected with the cerebrospinal center or axis. These are usually described in two classes—the spinal and the cranial or encephalic. The former class consists of all those which arise from the spinal cord, and emerge from the spinal canal through the inter-vertebral foramina; while the latter includes those which arise from some part of the cerebrospinal center, and emerge through foramina in the cranium or skull.