254 



FUNCTIONS OF THE NERVOUS SYSTEM. 



which is situated in the posterior part of the body, to the pedal ganglion of 

 each side, (c, c). Where such is the case, the trunk is not united with that 



Fig. 125. 



proceeding 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 gan- 

 glion is really made up of a pedal and 

 palleal ganglion, as 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 ap- 

 pears as a third. In the Bullsea, 

 whose nervous system is disposed on 

 the same general plan, the pedal and 

 palleal ganglia are separately connected 

 with the cephalic ; the cord from the 

 branchial ganglion passing through the 

 palleal. 



322. 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 cords 

 proceeding from the cephalic mass do 

 not lose themselves in the grey matter 

 of these ganglia; but divide themselves 

 into filaments, which mix with those 

 proceeding from them, to form the 

 nervous trunks which they distribute. 

 We can scarcely, then, fail to infer, 

 that the pedal ganglion, with the nerv- 

 ous fibrils proceeding from itself, is the 

 source of the reflex actions of this or- 

 gan; whilst the filaments which are con- 

 tinuous with those of the connecting 

 trunk, and which are thus connected 

 with the nucleus of the cephalic gang- 

 lia, are the channels of sensory impres- 

 sions, and of the motor impulses prompted by them. This is well illustrated 

 in the curious disposition of parts, which we find in the arms of the Cuttle-fish. 

 These are provided, it is well known, with a series of suckers, which are to 

 the animal important instruments of locomotion and prehension. It has been 

 observed by Dr. Sharpey, that the nerves which supply these arms are fur- 

 nished with ganglionic enlargements, of which one corresponds with each 

 sucker; and that each trunk consists of two tracts, in one of which the gan- 

 glionic enlargements exist; whilst the other passes continuously over these, 

 but sends off nervous filaments, which help to form the branches going to the 



Nervous system of Aplysia. A. pharyngeal gan- 

 glion ; B, cephalic ganglion. The cephalic is con- 

 nected, by three distinct cords on each side, with 

 the lateral masses, c c, 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 con- 

 necting cord passes backwards on each side to the 

 branchial ganglion, D ; this cord is continuous with 

 one of the three proceeding from the cephalic gan- 

 glion. 



