280 



COMMISSUEAL ACTION. 



Fig. 132, nervous system of aplysia : a, an- 

 terior ganglion ; c, cephalic ; I, I, lateral ; g, 

 abdominal. 



In the mechanical interpretation of the nerv- 

 Function of ous system, the action of commis- 

 commissures. SU ral strands is a point of primary 

 importance. It may Ibe said that they are for 

 the purpose of drawing off from the nerve arc a 

 part of the influence which is coming along the 

 centripetal fibre, and directing it into a new chan- 

 nel. If such coarse illustrations are permissi- 

 ble, the vesicles act like a three-way cock, or 

 perhaps like a piece of looking-glass with a 



part of the foil removed from its midst; a Nervous system of apiysia. 

 beam of light impinging upon it is in part reflected, and part escapes be- 

 hind through the uncovered space. Though I have described the simple 

 cellated nerve arc as containing essentially a ganglion or vesicle, it is not 

 to be supposed that such a structure necessarily impresses any change 

 on the incoming influence. Since, if we irritate a centripetal fibre, mus- 

 cular motion may ensue from propagation of that irritation through the 

 ganglion, and if we irritate a centrifugal fibre, muscular motion equally 

 ensues, it is quite clear that in the so-called action of reflection by the 

 ganglion there is, in reality, no change in the influence which has been 

 brought along the centripetal fibre. The same impression on any part 

 of the nervous arc, no matter on which side of the ganglion it may be 

 made, will produce the same muscular result. 



Such considerations therefore lead us to suspect that nothing takes 

 Actofreflec- place in the ganglion which justifies such an expression as 

 don. u ac t of reflexion" or "reflex action," terms which convey an 



idea that the influence which passes in the two branches of the nerve 

 arc is different, the difference having been established or brought on by 

 the ganglion. They confirm the opinion that the ganglion has, for one of 

 its primary duties, the function of permitting an escape of the influence 

 passing in the interior of the centripetal fibre into new channels for the 

 establishment of new results. 



In the simple automatic nerve arc the impression and the effect are in- 

 instantaneous stantaneous. An irritation of the centripetal branch pro- 

 action of the duces, without any sensible interval of time, muscular con- 

 traction through the action of the centrifugal branch, and that 

 contraction ceases the moment the impression is over. But the open- 

 Reserved action ing out of the nerve arcs by the introduction of a vesicle 

 of cellated arcs, permits a part of the influence, whatever it may be, to be 

 drawn off, and this, now passing along the commissural line, may be dis- 



