Nerve

{nurv) [nervus, nerve]. An elongated, cord-like structure made up of aggregations of N. -fibers and having the property of transmitting nervous impulses. N., Accelerator, the cardiac sympathetic N., stimulation of which causes acceleration of the heart’s action. N., Afferent, one that transmits impulses from the periphery to the central nervous system. N., Calorific, a N., stimulation of which increases the heat of the parts to which it is distributed. The term is applied to the dilator nerves and to those stimulating functional activity of organs. N.-cell, a ma of protoplasm containing a large vesicular nucleus within which lies a well-marked nucleolus. N. -cells have one or more elongated proce es, and in accordance with the number of these are designated unipolar, bipolar, or multipolar. The proce es are of two kinds: the axis cylinder proce and the protoplasmic proce es. The axis-cylinder (Deiter’s) proce either becomes an axis-cylinder of a N. -fiber, or divides within the gray matter into delicate filaments; it gives off minute branches termed collaterals; the other proce es are supposed to have nutritive and conducting functions. N. -center, a group of ganglion cells acting together in the performance of a function. N., Centrifugal. See N., Efferent. N., Centripetal. See N., Afferent. N. -corpuscles, nucleated corpuscles lying between the neurilemma and the medullary sheath of medullated N. -fibers. N., Cranial, a N. arising directly from the brain and making its exit through one of the foramina in the skull. N., Depre or, an afferent N., irritation of which depre es or inhibits the vasomotor center. N., Efferent, one carrying impulses from the central nervous system to the periphery. N.- endings, the terminations of nerves at the periphery or in the nerve-centers. N. of Expre ion, the facial nerve. N. -fiber, a fiber having the property of conducting invisible or molecular waves of stimulation from one part of an organism to another, and so establishing physiologic continuity between such parts without the nece ary pa age of waves of contraction (Romanes]. There are two kinds of nerve-fibers: the medullated, or myelinic, and the non medullated, or a myelinic. A typic medullated fiber consists of the axis-cylinder, which may be surrounded by a sheath, the axilemma; the medullary sheath, or white substance of Schwann; the neurilemma, or sheath of Schwann. The axis-cylinder is made up of fine fibrillae cemented together by a granular interstitial substance, the neuroplasm. The medullary sheath consists of a reticulated framework composed of a material resembling neurokeratin, and a semifluid substance filling the interstices, called myelin. At regular intervals the medullated sheath is marked by interruptions, the nodes of Ranvier. Each internode, as the portions of fiber included between two Ranvier” s nodes are termed, po e es a nerve-corpuscle. The neurilemma, sheath of Schwann, or primitive sheath, is a delicate elastic membrane, closely investing the medullary substance. On its inner surface are placed the nerve-corpuscles. The non medullated, pale, or Remak’s fibers do not po e a medullary sheath, but consist only of axis-cylinder and neurilemma. The nerve corpuscles are more abundant than in medullated nerve-fibers. Medullated nerve fibers are found in the cerebrospinal nerves, while non medullated fibers occur in the sympathetic nerves and tend to form plexuses. Xerve-fibers are aggregated into fasciculi; these in turn unite to form nerve-trunks. The nerve-fibers in a fasciculus are held TABLE OF NERVES Name. Function. Origin. ch506.xhtml a 434