NERVE-FIBRES

n. pi. Nu6rv'fi"b'rz. Lat.,yZbro? nervece, fila nerve a, tubuli nerve i. Fr., fibres nerve uses. Ger., Nervenfasern, Nervenfaden, Primitivfasern, Primitivrohren. The elongated thread-like elements of the nervous system. Each n.-f. consists of an axis-cylinder, and in most cases of a neurilemma and a medullary sheath. According to the presence or absence of the latter, the fibres are divided into two groups the white, or medullated, and the pale, or non-medullated, n.-f. The peripheral n.-f., except near their termination, are combined into bundles of various sizes by means of connective ti ue. [J, 10, 30, 31, 35.] Medullated n.-f., Myeliiiic n.-f. Fr., fibres nerve uses medulla ires (ou d myeline), tubes nerveux d myeline (ou d double contour, ou de la vie animale, ou Wanes). Ger., doppeltconturirte Nervenfasern. Markfasern. N.-f. with a sheath of myelin inclosing the axis-cylinder. They are found only in the vertebrates, with the exception of some Crustacea. In the peripheral medulEo ibleited n.-f. there is present also a neurilemma, so that they consist of at least three parts, a central axis-cylinder, inclosed by the medullary sheath, which in turn is inclosed by the neurilemma. In these nerves the medullary sheath is partly or completely interrupted at regular intervals, with a corresponding constriction of the neurilemma, forming the nodes of Ranvier. In the medullated nerves of the central nervous system and of the optic nerve the pre sO, no; O, not; O3, whole; Th, thin; Th, the; U, like oo in [s. 93]