252 FUNCTIONS OF THE NERVOUS SYSTEM. 



careful dissection, that these cords do not serve merely to bring the ganglia 

 into relation : but that a part of them pass through the ganglion into the trunks 

 proceeding from it. Thus, of the nerves which supply the large fleshy foot, 

 and which appear to proceed from the pedal ganglion, 6, a part are undoubt- 

 edly connected with that ganglion alone, coming into relation with its vesicu- 

 lar substance ; but a part also pass on to the cephalic ganglia, by the connecting 

 trunks, so that these, rather than the pedal ganglion, constitute their centre. 

 The same may be said of the nerves proceeding from the branchial ganglion: 

 a portion of them having their centre in the vesicular matter of that ganglion; 

 whilst another portion has no relation to it whatever (beyond that of prox- 

 imity), but passes through or over it, to become connected with the cephalic 

 ganglia. There is good reason to believe, that the pedal and branchial gan- 

 glia minister to the purely reflex actions of the organs they respectively sup- 

 ply; and that they would serve this purpose as well, if altogether cut off from 

 connection with the cephalic ganglia : whilst the latter, being the instruments 

 of the actions which are called forth by sensation (whether these be of a con- 

 sensual or of a voluntary nature), exert a general control and direction over 

 the movements of the animal. 



319. The animals of the class GASTEROPODA, whether furnished with uni- 

 valve shells, or entirely destitute of such protection, are, for the most part, 

 much more highly organized than the preceding; possessing not merely 

 greater locomotive power, but organs of special sense, which are situated in 

 the neighbourhood of the mouth, upon a projecting part of the body, which is 

 thus constituted a head. Their nervous system consists of at least three dis- 

 tinct centres ; the relative position of which varies with that of the organs 

 supplied by them. The anterior or cephalic ganglia are larger in proportion 

 to the rest, than they are in the Conchifera ; and they exhibit a tendency to 

 gain a position anterior to the oesophagus, and to approximate towards each 

 other, so as to meet and form a single ganglionic mass on the median line. 

 The branchial ganglion is constantly to be met with ; but its position is ex- 

 tremely variable. This centre, however, always bears a close relation with 

 the gills, both in situation and in degree of development; and even where 

 conjoined, as it frequently is, with the pedal ganglion, it may be distinguished 

 from it by the distribution of its nerves, as well as by its separate connection 

 with the cephalic ganglia, which is always noticed in such cases. This may 

 be observed in the Patella (limpet) and Limax (slug). Sometimes the func- 

 tions of this ganglion are subdivided between two ; of which one is still ap- 

 propriated to the branchiae ; whilst the other is connected with the general 

 surface of the mantle, and with the respiratory passages which are prolonga- 

 tions of it, and hence may be called the pallcal ganglion. The position of 

 the pedal ganglion (which is generally double in the Gasteropoda, though the 

 foot is single), also varies, but in a less degree, since it is generally in the 

 neighbourhood of the head. Besides these nervous centres, we find, in many 

 of the Gasteropoda, a separate system connected with a very important set 

 of organs, the gustatory and manducatory, which are but slightly shadowed 

 out among the Conchifera. In these higher tribes, the oesophagus is dilated 

 at its commencement into a muscular cavity (Fig. 3, a]; containing a curious 

 rasp-like tongue, often supported upon cartilages, which serves to reduce the 

 food; and sometimes furnished with horny maxilla?. The nerves which sup- 

 ply these do not proceed directly from the cephalic ganglia, but from a dis- 

 tinct centre; and their ramifications proceed along the oesophagus and sto- 

 mach, and are occasionally connected with the other nerves by inosculating 

 filaments. This set of ganglia and nerves, which is even more important 

 from its relative development in some other classes, and into the analogies of 



