Nerve (Gir. vevpov, a string or sinew), a white cord-like handle of filaments, distributed to asitive or contractile organs of the body, and capable of transmitting the nervous influence; so called because the Greek anatomists, misled by the aspect of the nerves proper, did not distinguish them from the tendons. Externally a nerve is white and glistening, and of considerable toughness and consistency. These qualities are due to its being covered everywhere with a layer of white fibrous tissue, of the same kind as that of the tendons and ligaments, which serves to support the softer parts within and protect them from injury. This protective investment is termed the neurilemma. It sends everywhere longitudinal partitions into the interior of the nerve, in which are contained the small blood vessels destined to nourish its tissue, dividing it into a number of parallel passageways or channels, of a nearly cylindrical form. In these channels are contained the nervous filaments or nerve fibres, the essential anatomical elements of the nerve.
They are cylindrical filaments, averaging in the main trunks and branches 1/2000 of an inch in diameter, and consisting of a fine structureless investing membrane, a layer of semi-fluid, transparent, highly refracting substance, the "medullary layer," and a central, soft, faintly granular mass, the "axis cylinder." They are similar to the nerve fibres in the brain and spinal cord Brain), except that they are larger in size, and are invested by the fibrous neurilemma, which is wanting in the interior of the nervous centres. The presence of the highly refracting medullary layer gives to each filament a distinctly marked double contour, which renders it easily distinguishable under the microscope, The filaments in the interior of the nerve are thus arranged in parallel bundles, each bundle surrounded by its own layer of white fibrous tissue, and the whole surrounded and strengthened by the external investment of neurilemma. As the nerves, after originating from the brain or spinal cord, pass outward toward the organs to which they are to he distributed, they divide into smaller and smaller branches, and sometimes send to each other reciprocal branches of communication, thus forming nervous plexuses, which have received distinct names, corresponding with their location.
Thus we find the cervical plexus in the neck, and the brachial plexus, from which are given off the nerves going to the arm. But in these cases the branching and intermingling of the nerves is only apparent, and is simply due to the separation of certain bundles of filaments from those with which they were previously associated, and their passing off in a different direction. The nervous filaments themselves do not in these instances split up or lose their identity. But when a nerve has finally reached the organ in which it is to be distributed, and when by successive ramification its branches have become reduced to a few filaments each, these filaments themselves divide and multiply, perhaps several times in succession, and often without diminishing very perceptibly in size, although at the point of division they usually exhibit a well marked constriction. The nervous filaments finally terminate by free extremities, both in the muscular and sensitive tissues. In the voluntary muscles the terminal extremity of a nervous filament becomes attached to a muscular fibre, its investing tubular membrane becoming continuous at the point of junction with the sarcolemma, its medullary layer disappears, and its axis cylinder spreads out upon the surface of the contractile substance of the muscular fibre, in the form of a thin, granular, oval spot.
In the skin and sensitive membranes generally the ultimate nervous filaments end in minute rounded or ovoid masses termed tactile corpuscles, within which they lose, as in the case of the muscular nerves, their investing membrane and medullary layer, and become reduced to the axis cylinder alone, which is usually terminated by a slightly rounded extremity. - Nerves are divided into motor and sensitive, according to the preponderance of the two kinds of nervous filaments in their tissue, and whether they are distributed to muscular or sensitive organs. A nerve distributed to muscles is a motor nerve, and its irritation produces a muscular contraction; one distributed to the integument is a sensitive nerve, and its irritation causes a painful sensation. Thus the seventh pair of cranial nerves, or the facial, belongs to the motor nerves, and animates the superficial muscles of the face. The fifth pair, on the contrary, belongs to the sensitive nerves, and communicates sensibility to the integument of that part of the body. la point of fact, however, but few if any of the nervous trunks are exclusively motor or exclusively sensitive, since they generally receive filaments of both kinds, either from their own roots or from other neighboring nerves.
Thus the facial nerve has a certain degree of sensibility, which it derives from communications with the fifth pair; and one portion of the fifth pair itself is motor in character, and animates the muscles of mastication. When both kinds of filaments are mingled together in a nerve in similar or nearly similar proportions, it is said to be a "mixed nerve," and is at the same time motor and sensitive. This is the case with all the spinal nerves, the branches of which are distributed both to the muscles and the integument of the body and limbs. - When cut across, a nerve at once ceases to perform its functions. It can no longer transmit the nervous influence in either direction, and accordingly the parts to which it is distributed become paralyzed and insensible. The nerve may however reunite after such an injury, and its natural functions thus become reestablished. The substance may even be regenerated when a considerable portion has been cut out. This reunion and regeneration of divided nerves takes place most readily in young animals.
Vulpian has found the sciatic nerve regenerated, in very young rats, after the excision of a little more than two inches of its length, in 17 days; and in young cats sensibility has returned in the tongue, after excision of one inch of the lingual nerve, in 14 days. In adult animals, however, and in the human subject, a longer time is required for the regeneration of a divided nerve; and the full restoration of its function is often not complete until after the lapse of several months. (See Nervous System).
Fig. 1. - Transverse Section of the Iselriatic Nerve. a. Neurilemma, ft. Internal Fibrous Partitions, c. Bundles of Nervous Filaments, cut across.
Fig. 2. - Division of Nervous Filaments, from one of the Cutaneous Muscles of the Frog, magnified 350 times.