HISTOLOGICAL ELEMENTS 



827 



fibrils from cell to cell has 

 been demonstrated in some 

 of the invertebrates e.g., 

 in annelids (Fig. 321) 

 where previously the best 

 examples of strictly iso- 

 lated neurons were sup- 

 posed to be found (Apathy) . 

 The supporters of the 

 theory of continuity look 

 upon the cell-body as 

 merely necessary for the 

 nutrition of the nerve-net, 

 but deny that it is neces- 

 sary for the conduction of 

 nerve-impulses. If this is 

 the case, it is obvious that 

 the neurons can no longer 

 be considered as functional 

 units in which the law of 

 isolated conduction of 

 nerve-impulses (p. 767) 

 holds good. Nor is it by 

 any means so easy to un- 

 derstand as on the neuron 

 hypothesis such facts as the 

 strict limitation of Wal- 

 lerian degeneration to the 

 boundaries of the neurons 

 directly affected, or the 

 strict limitation of the 

 silver reduction in Golgi 

 preparations to single neu- 

 rons. It is, of course, true 

 that the simplicity and 

 order introduced by the 

 neuron hypothesis into our 

 conceptions of the nervous 

 conduction paths by no 

 means prove its accuracy. 

 Yet they are reasons for 

 not lightly abandoning it, 

 and it has recently been 

 corroborated by important 

 new evidence on the growth 

 of nerve-cells on artificial 

 media outside of the body 

 (p. 776; Fig. 327, p. 829). 

 Varieties of Neurons. 

 Nearly all the nerve-cells 

 of the cerebro-spinal axis 

 agree with the cells of the 

 anterior horn in the posses- 

 sion of an axon and one or 

 more dend rites, although 

 sometimes the dendrites 

 are scanty in number and 



Fig. 325. Scheme of Lower Motor Neuron 

 (Barker), a, h, axon-hillock (the portion of the 

 cell from which the axon comes off), containing 

 no Nissl bodies, and showing fibrillation; ax. 

 axis-cylinder or axon; m, medullary sheath, 

 outside of which is the neurilemma; c, cell- 

 substance (cytoplasm), showing Nissl bodies in 

 a lighter ground substance; d, protoplasmic 

 processes or dendrites containing Nissl bodies; 

 n, nucleus; n', nucleolus; n, R, node of Ranvier; 

 s,/, side fibril; not^n, nucleus of the neurilemma ; 

 tel., motor end -plate; m', striped muscle-fibre; 

 s, /., incisure. 



