6o6 



THE NERVE CELL. 



towards the attachment of the axis cylinder and leave the cell by that 

 process. . In other words, that the direction of conduction in the dendrons 

 is centripetal, in the axon centrifugal ; this has been termed the " law 

 of axipetal conduction." This is included in the more general state- 

 ment by W. James, 1 that nerve impulses take always a particular 

 direction in the nervous system, and never naturally the reverse 

 direction (" law of forward direction ") : that there is, in short, a kind 

 of valved action which permits passage of impulses in one direc- 

 tion only. 



The law of axipetal conduction is no doubt true for many cells, but 

 it will not hold good for the typical sensory cell. This, in its simplest 



and original form, is a 

 bipolar cell with two 

 axons and no dendrons, 

 and the nervous im- 

 pulses produced at the 

 periphery pass towards 

 the cell by one axon, 

 and away from it by the 

 other. The contention 

 that the peripheral pro- 

 cess, although possess- 

 ing all the characters 

 of an axon, must be 

 considered a dendron, 

 because it conducts cen- 

 trifugally, merely begs 

 the question. 



Although nothing 



whatever has been as- 



Fig. 307.— Diagram to illustrate certained regarding the 



supposed lines of conduction na t U re of the change 



in a spinal ganglion cell. — a, , . . . 9 



body of cell f b, its axis which IS Set Up 111 



cylinder process ; c, fibre of the nerve element, and 



posterior root; d, peripheral which travels along it as 

 fibre ; e, termination of axis ■, , • n j j.i_ 



cylinder of another cell (?sym- what 1S called the nerv " 



pathetic) ; m, ramifications of OUS impulse, it is clear, 



nerve fibre in spinal cord ; p, f rom an examination of 



ramifications at sensory sur- , i i , • ■> 1 



face.-Cajal. the . electrical change 



which accompanies its 

 passage along a nerve, that it passes in the form of a wave, rising at any 

 point of its course to a certain intensity, and gradually falling from this 

 crest to a point at or even slightly below the normal. It would further 

 appear, from observations conducted by the same method upon the 

 nerve centres, that if the body of the cell is stimulated by a prolonged 

 or intense stimulus, the nervous impulses which are discharged tend, 

 even if the stimulus be uninterrupted, to become discontinuous ; in 

 other words, to exhibit a rhythm of a more or less regular character. 

 Such rhythm can be also investigated by the effect it produces upon 

 the muscles which are connected with the nerve elements in activity. 

 Tracings of muscular contraction, or of the changes in electrical potential 

 which accompany the contraction, exhibit a curve with waves corre- 



1 "Principles of Psychology," 1890, vol. ii. 



Fig. 30G. — Diagram to il- 

 lustrate the "law of 

 axipetal conduction" in 

 motor nerve cells. A, 

 axis cylinder process. 

 B, nucleus ; a, fibrils of 

 cell body ; b, c, e, Nissl 

 bodies; d, fibrils of den- 

 dron. The arrows indi- 

 cate the direction of 

 nerve impulses. — Cajal. 



