Page:The New International Encyclopædia 1st ed. v. 14.djvu/418

* NERVOUS SYSTEM AND BRAIN. 368 NERVOUS SYSTEM AND BRAIN. its liiicuess, its straight course, approximately uuiform diameter, and its few branohos. These few biuiK-hes pass off at right angles and are known as collaterals. In sonic cells the axone branches rapidly and ends in the gray matter near its cell of origin. Other axones pass into the white matter and become axis cylinders of nerve (ibres. At its origin from the cell body and at its termination, the axone is uncovered by any sheath. Some axones — confined to the gray matter — are entirely devoid of any covering. Other axones — fibres of Remak. found mainly in the sympathetic system — are covered by a deli- cate sheath known as the neurilemma or sheath of Schwann. The axones of the white matter of the brain and cord are jjro- tected by a thick sheath called the medullary sheath. The axones of the periplieral cranial and spinal nerves arc covered by a medullary sheath anil outside of this a neurilemma. Such axones, with their sheaths, are known as medullated nerve fibres. A medullated nerve fibre thus consists of a central core or axis cylinder which is contiinious fliroughout the fibre and is the axone of a nerve cell. This axis cylinder is cov- ered byarather thick sheath of a fatty nature known as the medullary or myelin slieath. outside of which is the delicate neurilemma. From tlu> neurilenuna fine septa extend into the myelin called tlu' in- cisures of Schmidt. At intervalsthe med- ullary sheath is in- terrupted and the axis cylinder is un- covered or covered only by the neuri- lemma. These points are known as the nodes of Ranvier, and the portion of the nerve fibre between two nodes as an internodal segment. As to the physiological significance of these dilTerent parts of the neurone, our ideas are still based hugely upon theoretical grounds. The cell body has been shown to 1h" the birth or genetic centre of the neurone. Krom the fact that any portion of the neurone which is cut ofT from the cell body dies, the cell would seem to be the nutritive or trophic centre of the neurone. From the absence of nervous activity in portions of the neurone which have been cut olT from their cell of origin, the cell body would also appear to be the functional centre of the neurone. The most MEDULLATBD NERVE FIDRKS (08MIC AC1I>). .. VIpw. B. .Section, lijin. no'l.. RanviiT'H iioilfH, where the ineduliar.v sheatli is internipted. and the axis (•.vlliuliT iippfiirH; Ax. c.r/.. A.xis cylinder: tn.Sclim., liU'iHiireK (if Si'liinidt : (int. mth., (fruniiinr substance nt the poles n( tht' nueletiH; Stiv.. .Nucleus; Sh. Svh., Sheath of Schwann. generally accepted theories of the significance ot the component parts of the cell itself may be summed up as follows. The function of the nucleus is the same as in other cells. It seems to jjreside over the constructive activities of the neurone, by which food products brought to the cell by the lymph are transformed into food elements of the cell, and finall.v into its nervous elements proper. The basement substance and the cliromophilic bodies represent the food ele- ments of the cell. The cytoreticulum represents the working nervous mechanism of the cell, it alone lieing concerned in the reception, trans- formation, and emission of the nervous impulse. Of the processes the axone is centrifugal in func- tion, carrying impulses awav from the cell body, the dendrites are centripetal in function, bring- ing impulses to the cell body. Axones thus act as organs of distribution as regards the nervous impulse, dendrites as organs of reception. Neu- rones are associated with one another by approxi- mation or bv contact, and not liv continuity of their protoplasm : that is. no two neurones are believed to be diiectl.v connected with each other. The axonal terminations of one neurone siuipl.v lie in contiguity, or at most in touch with the dendrites or cell bodies of other neurones, the impulse passing over from one to the other. Neuroglia, or the connective tissue of the nervous svstem, difl'ers both in structure and in origin from the form of connective tissue found in other organs. .s alrcadv stated, it is epi- blastic in origin, dcveloiiing like the neurone from the cells which line the embrvonic ni'ural canal. These cells, at first morphologically iden- tical, soon differentiate into neuroblasts, or future neurones, and spongioblasts, or future neuroglia cells. In adult neuroglia two main t.vpes of cells arc found, spider cells and mossy cells. The spider cell has a rather small body, from which are given oir on all sides straight, unbranchiiig, s]iine- like processes. They occur mainly in the white CEI.L8 FROM SECTIONS OF THE DRAIN OF ADULT MAN. a. Mossy cell; b, spider cell. matter. The mossy cell has also a rather small bodv, its processes are fewer, arc coarse, rough, and brnnching; they occur mainly in connection with blood-vessels. As in the case of the nerve cell, the jirocesscs of these cells do not anasto- mose, but interlace, forming a dense feltwork. The nervous system is composed in all ver- tebrated animals of two distinct portions or systems — viz. the ccrcbro-spiniil and .ii/iiiixilliclic or (iiintilirmic. The cerebrospinal si/stem includes the brain and spinal cord (which form the rcrrhronpinnl axis, or central nervous .system) and the cranial and spinal nerves. It was termed bv Bichat the nervous system of animal life, and comprises all