Page:The American Cyclopædia (1879) Volume XII.djvu/240

 232 NERVAL his reign a conspiracy was formed against him, at the head of which was a descendant of the triumvir Crassus. It was discovered in time ; but Nerva having sworn, when accepting office, that no senator should suffer death under his rule, the leader was only hanished to Tarentum and the other conspirators were pardoned. NERVAL, Gerard de. See GERARD DE NERVAL. NERVE (Gr. vevpov, a string or sinew), a white cord-like bundle of filaments, distributed to sensitive 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 dis- tinguish them from the tendons. Externally a nerve is white and glistening, and of consider- FIG. 1. Transverse Section of the Ischiatic Nerve. a. Neurilemma. 6. Internal Fibrous Partitions, c. Bundles of Nervous Filaments, cut across. able 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 in- vestment is termed the neurilemma. It sends everywhere longitudinal partitions into the in- terior of the nerve, in which are contained the small blood vessels destined to nourish its tis- sue, dividing it into a number of parallel pas- sageways or channels, of a nearly cylindrical form. In these channels are contained the nervous filaments or nerve fibres, the essen- tial anatomical elements of the nerve. They are cylindrical filaments, averaging in the main trunks and branches 2 ^ 6 of an inch in diame- ter, and consisting of a fine structureless invest- ing membrane, a layer of semi-fluid, transpa- rent, highly refracting substance, the " medul- lary layer," and a central, soft, faintly granular mass, the "axis cylinder." They are similar to the nerve fibres in the brain and spinal cord (see 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 dis- tinctly 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 strength- ened by the external investment of neurilemma. As the nerves, after originating from the brain or spinal cord, pass outward toward the organs NERVE to which they are to be distributed, they divide into smaller and smaller branches, and some- times send to each other reciprocal branches of communication, thus forming nervous plex- uses, which have received distinct names, cor- responding with their location. Thus we find the cervical plexus in the neck, and the brachi- al 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 separa- tion 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 be- come reduced to a few filaments each, these filaments themselves divide and multiply, per- haps 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 ner- vous filaments finally terminate by free ex- tremities, both in the muscular and sensitive FIG. 2. Division of Nervous Filaments, from one of the Cu- taneous Muscles of the Frog, magnified 350 times. tissues. In the voluntary muscles the terminal extremity of a nervous filament becomes at- tached 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 sub- stance of the muscular fibre, in the form of a thin, granular, oval spot. In the skin and sensi- tive membranes generally the ultimate nervous filaments end in minute rounded or ovoid mass- es 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,