Nerve
or NERVOUS SYSTEM. A nerve is one of the fibers which proceed from the brain and spinal cord, or from the central ganglia of lower animals, and ramify through all parts of the body, and whose function is to convey impulses resulting in sensation, motion, secretion, etc. The aggregate of these nerves, and the centers from which they proceed, forms the nervous system, the medium through which every act or detail of animal life is inaugurated and directed. The essential idea of any nervous system involves the necessary presence, firstly, of a nerve center or centers, which generate the nervous force or impulse; secondly, of conducting fibers or cords, the nerves; and thirdly, of an organ, part, or structure to which the impulse or impression may be conveyed. The nerve-centers of man and vertebrates generally are disposed so as to form two chief sets, which are to be regarded as essentially distinct.
The brain and spinal marrow together (see BRAIN) constitute the first of these centers, and are collectively included under the name cerebro-spinal system or axis. The second system is the sympathetic or ganglionic. From each of these systems nerve-cords are given off-the cerebral and spinal nerves from the former; and the socalled sympathetic fibers from the latter. The brain and spinal cord are contained within the continuous bony case and canal formed by the skull and spinal column; while the chief masses of the sympathetic system form an irregu larly disposed chain, lying in front of the spine, and contained within the cavities of the thorax or chest and abdomen. The general functions of the cerebro-spinal system system are those concerned with volition and muscular movements, with the control of the senses, and in higher forms with the operations of the mind. The nerves of the sympathetic system in chief are distributed to the viscera, such as the heart, stomach, intestines, blood-vessels, etc.; and the operation of this system is in greater part of involuntary kind, and without the influence or command of the will. The cranial or cerebral nerves pass from the brain through different openings in the skull, and are all in pairs, the first pair being the olfactory nerves or nerves of smell; the second, the optic nerves, or nerves of sight; while others have to do with hearing, taste, general sensibility, and muscular motion. The spinal nerves, after issuing from their openings in the vertebral column, split into two divisions, one of which proceeds to supply parts NESS behind the spine, while the other passes toward the front. The first eight spinal nerves on each side are called cervical, the next 12 are dorsal, the next five lumbar, then five sacral, and one coccygeal. one The general functional relation of the nervous system may be summarized by stating that its functions comprehend the reception and distribution of impressions; that these impressions originate either from influences acting on the periphery, or from the nerve-centers, brain, or mind; these impressions respectively influence or stimulate the mind or nerve-centers, and the muscles or secreting structures; and lastly, that all nervous phenomena are exerted through or accompanied by nervous action, and that this latter is, so far as physiology has yet been able to determine, of a uniform and similar kind. See also EYE; EAR; NOSE; etc. such The Invertebrata possess no specialization of the nervous centers as is seen in Vertebrates, in which the brain and spinal cord are inclosed within their bony case and canal, and thus shut off from the general cavity of the body. The great and distinctive feature between the nervous system of Vertebrata and that of the lower forms consists in the absence of a defined or chief nervous center, through which consciousness may intervene to render the being intelligent, and aware of the nature of the acts it performs.