GENERAL PHYSIOLOGY OF NERVE-TISSUE. 105 



drites, though forming an intricate feltwork, never anastomose with 

 one another nor unite with dendrites of adjoining cells. According 

 to the number of axons, nerve-cells are classified as monaxonic, 

 diaxonic, polyaxonic. VMost of the cells of the nervous system of the 

 higher, vertebrates are monaxonic. In the ganglia of the posterior 

 or dorsal roots of the spinal and cranial nerves, however, they are 

 djaxonic.\< In this situation the axons, emerging from opposite poles 

 o?"the cell, either remain separate and pursue opposite directions, or 

 unite to form a common stem, which subsequently divides into two 

 branches, which then pursue opposite directions. (See Fig. 43, B.) 

 The nerve-cell maintains its own nutrition, and presides over that of 

 the dendrites and the axon as well. If the latter be separated in any 

 part of its course from the cell, it speedily degenerates and dies. 



The axon, or nerve process, arises from a cone-shaped projection 

 from the surface of the cell, and is the first outgrowth from its pro- 

 toplasm. At a short distance from its origin it becomes markedly 

 differentiated from the dendrites which subsequently develop. It 

 is characterized by a sharp, regular outline, a uniform diameter, and 

 a hyaline appearance. In structure, the axon appears to consist of 

 fine fibrillas embedded in a clear, protoplasmic substance. Shafer 

 advocates the view that the fibrillae are exceedingly fine tubes filled 

 with fluid^The axon varies in length from a few millimeters to one 

 meter. /, In the former instance the axon, at a short distance from its 

 'origin, divides into a number of branches, which* form an intricate 

 feltwork in the neighborhood of the cell. In the latter instance 

 the axon continues for an indefinite distance as an individual struc- 

 ture. In its course, however, especially in the central nervous system, 

 it gives off a number of collateral branches, which possess all its his- 

 tologic features. The long axons serve to bring the body of the cell 

 into direct relation with peripheral organs, or with more or less re- 

 mote portions of the nervous system, thus constituting association or 

 commissural fibers. 



The more or less elongated a^abecomes invested, as a rule, at a 

 short distance from the cell with nucleated oblong cells, which subse- 

 quently become modified and constitute the ^rl^fliy or myelin 

 sheath. This is invested by a thin, cellular membrane -the" fleu- 

 rilemma. >J?These three structures thus constitute what is known as a 

 'meMUliated nerve-fiber^ In the central nervous system the outer 

 sheath is frequently absenfTyln the sympathetic system the myelin 

 is also frequently absent, though the axon is inclosed by the neuri- 

 lemma, thus constituting a non-medullated nerve-fiber.^ 



The end-tufts or terminal organs are formed by the splitting' of the. 

 axon into a number of filaments, which remain independent of one 

 another and are free from the medullary investment. The histologic 

 peculiarities of the terminal organs vary in different situations, and in 



