REGISTERING NERVE ARC. 



281 



posed of in two different ways : 1st. The influence thus drawn off may 

 be instantaneously consumed or utilized by exciting, through adjacent 

 simple arcs, synchronous movements ; or, 2d. It may be held in reserve 

 for future use by being carried along the commissure to a receiving, or, 

 as I may term it, registering ganglion. 



This, therefore, introduces a more complex mechanism, which may be 

 designated as, 



The registering nerve arc, the typical construction of which is repre- 

 133. sented in Fig. 133. In this we have the Registering 



centripetal, a, and centrifugal fibre, 6, as be- nerve arcs - 

 fore, in connection with their central vesicle, v ; but, 

 passing from that central vesicle, a commissural fibre, 

 c, offers a channel of escape of a part of the influence 

 which so reaches the registering ganglion, r, and makes 

 a permanent impression upon it by disturbing its con- 

 dition physically or chemically; and, since many nerv- 

 ous arcs may be thus commissured upon the same registering ganglion, it 

 thus becomes for them all a central point of deposit and a centre of com- 

 mon action. And in this manner not only is a temporary Variable ef- 

 influence converted into a permanent impression, but, from f ectsansefrom 



x _ invariable im- 



the interaction of such impressions upon one another, new pressions. 

 and variable results arise. Some illustrations were given a few pages 

 back of the development of the variable from the invariable in the case 

 of certain ordinary physical phenomena, and these may be profitably re- 

 ferred to again. 



A modification of the registering nerve arc is presented in Fig. 134, 

 Fig. is*, which exhibits the suppression of the centrifugal gu ression of 

 branch, the whole influence received passing along centrifugal 

 the commissural line to the registering ganglion. branch - 

 This condition of things may occur when the centripetal branch 

 at its free extremity is involved in a mechanism of special 

 sense, olfactive, ophthalmic, or auditory. No part of the im- 

 pression thus received is necessarily expended at once ; the 

 whole may be thus retained, and utilized at a future time, 

 gai branch. The introduction of the registering ganglion is thus the in- 

 troduction of the element of time in a living mechanism. In the lower 

 forms of arc an impression is instantaneously expended, in this it is pre- 

 served. 



The common centre or register of whatever impressions have been re- 

 ceived by the special sense instruments, olfactive, ophthalmic, or auditory, 

 as well as impressions of a general tactile kind, is doubtless 

 to be properly regarded as the sensorium. Though animals 

 constituted on this type accomplish many variable actions, that variabil- 



suppresson 



of centrifu- 





