The Nervous System

Neurology

The nervous system is a complex mechanism by which the organism is brought into functional relation with its environment, and its various parts are coördinated. For purposes of gross description it is divided primarily into two parts, central and peripheral.

The central nervous system (Systema nervorum centrale) comprises (a) the spinal cord (Medulla spinalis), and (b) the brain (Encephalon).

The peripheral nervous system (Systema nervorum periphericum) includes (a) the cerebral and spinal nerves with their ganglia, and (b) the sympathetic nervous system.

The division into central and peripheral parts is quite arbitrary and is employed purely as a matter of convenience of description. The fibers of which the nerves are composed either arise or end within the central system, and therefore constitute an integral part of the latter. The structural and functional unit of the nervous system is the neurone, which consists of the cell-body, usually termed the nerve-cell, and all its processes. The processes arise as outgrowths from the cell-body and conduct impulses to or from the cell. They vary greatly in length, some being less than a millimeter long, while others extend from a cell in the spinal cord to the distal end of a limb. A nerve is composed of such processes, usually inclosed in a protecting and insulating sheath, and united into bundles by connective tissue. The neurones are embedded in a peculiar supporting tissue termed neuroglia. This consists of a very intricate feltwork of glia-fibers, many of which are connected with the small glia-cells or astrocytes. In addition the nervous tissue is invested closely by a vascular layer of connective tissue, the pia mater, from which ingrowths extend into the nervous substance proper.

To the naked eye the central nervous system appears to be composed chiefly of two kinds of substance, white and gray. The white matter (Substantia alba) is dead white in color and is soft in the natural state. It consists largely of medullated nerve-fibers, packed closely together and arranged more or less clearly in large or small bundles or tracts (Funiculi, fasciculi). The gray matter (Substantia grisea) is usually brownish-gray in color, often with a tinge of pink. It is softer than the white substance and much more vascular. It is composed chiefly of cell-bodies and non-medullated processes. In some situations it is modified to form the gelatinous substance (Substantia gelatinosa), which is pale yellowish-gray and jelly-like.

Ganglia are gray masses found on the dorsal roots of the spinal nerves and on the course of many nerves. They are commonly ovoid in form and are inclosed in a fibrous capsule. They are composed largely of the cell-bodies of neurones, but have connected with and passing through them nerve-fibers (processes) which extend peripherally and centrally. In origin and function the spinal ganglia belong properly to the central system, but it is customary to include them with the peripheral part in gross anatomical descriptions.

Nerves are conducting trunks composed of bundles of parallel nerve-fibers. They are enveloped in a fibro-elastic sheath, the epineurium, which contains the blood and lymph vessels. They are classified according to their central connections as cranial or cerebral, spinal, and sympathetic. The groups of cells of which the nerve-fibers are processes are termed the nuclei of origin or termination of the nerves, according as the latter conduct impulses in a peripheral or central direction. On the same basis the nerve-fibers are designated efferent and afferent respectively. A bundle of fibers which passes from one nerve-trunk to another is called an anastomotic branch (Ramus anastomoticus). In some situations the exchange of branches between adjacent nerves is so free as to constitute a nerve-plexus (Plexus nervorum). The term ramus communicans is properly restricted to branches which connect the ventral divisions of the spinal nerves with adjacent ganglia or nerves of the sympathetic system. The terminal twigs of the nerves are designated according to their distribution as muscular branches (Rami musculares), cutaneous nerves (Nervi cutanei), and articular nerves (Nervi articulares). The muscular branches are motor in function, the cutaneous and articular sensory, but all contain vasomotor fibers which control the caliber of the blood-vessels.