108 



FUNCTIONS OF THE NERVOUS SYSTEM. 



Fig. 10. 



Nervous system of Aplysia. The most anterior 

 ganglion is the pharyngeal; and below this is seen 

 the cephalic. The cephalic is connected, by three 

 distinct cords on each side, with the lateral masses, 

 which combine the functions of pedal and palleal 

 ganglia; these are united with each other by two 

 transverse bands, between which the aorta passes. 

 From the lateral ganglia, a connecting cord passes 

 backwards on each side to the branchial ganglion ; 

 this cord is continuous \vith one of the three pro- 

 ceeding from the cephalic ganglion. 



enable them to act in harmony. This 

 is an inquiry of much interest, in re- 

 ference to the determination of the 

 offices of the different parts of the 

 nervous centres in Vertebrated a^i- 

 mals. If we examine the mode in 

 which the different ganglia are united 

 by connecting trunks, we are led to 

 perceive the important fact that, while 

 they have little or no communication 

 with each other, they are all directly 

 connected with the cephalic ganglia ; 

 which seem thus to harmonize and 

 control their individual actions. Fre- 

 quently a communication with one 

 another appears to exist, where there 

 is really none. Thus, in the Aplysia, 

 a cord passes from the branchial gan- 

 glion, which is situated in the pos- 

 terior part of the body, to the pedal 

 ganglion. Where such is the case, 

 the trunk is not united with that pro- 

 ceeding from the ganglion through 

 which it passes ; but the two remain 

 distinct, though running in the same 

 direction. Moreover, the double func- 

 tion of a ganglion may be sometimes 

 recognized, by its being connected 

 with the cephalic mass by a double 

 trunk. Thus, in the Aplysia, that 

 which has been termed the pedal 

 ganglion is really made up of a pedal 

 and palleal ganglion, as it is proved 

 by the distribution of its branches; 

 and in conformity with this double 

 function, we find it communicating 

 with the cephalic mass by two cords, 

 besides the one which has been just 

 mentioned as passing through it, and 

 which appears as a third. In the 

 BulldRa, whose nervous system is dis- 

 posed on the same general plan, the 

 pedal and palleal ganglia are sepa- 



rately connected with the cephalic; 

 the cord from the branchial ganglion passing through the palleal. 



139. Further, a careful examination of these ganglia, and of their connecting 

 cords, discloses this important fact, which is peculiarly evident in the case of 

 the pedal ganglia that the cord does not lose itself in the gray matter of the 

 ganglion, but divides itself into filaments, which mix with those proceeding 

 from it, to form the nervous trunks which it distributes. We can scarcely, 

 then, fail to infer that the pedal ganglion, with the nervous fibrils proceeding 

 from itself, is the source of the reflex actions of this organ ; whilst the filaments 

 which are continuous with those of the connecting trunk, and which are thus 

 connected with the nucleus of the cephalic ganglia, are the channels of sensory 

 impressions, and of the motor impulses of volition or instinct. This is well 



