104 FUNCTIONS OF THE NERVOUS SYSTEM. 



sensitive, and are sometimes increased in number to six or eight ; and there is 

 reason to believe that some of them occasionally minister to the sense of smell. 

 These senses, as well as the locomotive powers of the animals, have an obvious 

 relation with the supply of the digestive system ; which is not here, as in the 

 inferior classes, dependent upon the miscellaneous aliment conveyed to the 

 mouth by the movement of the surrounding fluid medium, but is more limited 

 as to the character of the food to which it is adapted ; so that the animal re- 

 quires the means of becoming acquainted with the proximity of what it can 

 digest. 



134. It is not a little curious, however, that, although the general surface 

 appears highly susceptible of impressions which excite responsive movements 

 adapted to fulfil some important office in the economy, it does not seem to be 

 susceptible of painful impressions, in any thing like the same degree. This, 

 which cannot but be regarded as a beneficent provision for the happiness of 

 animals so incapable of offering any active resistance to injury, would appear 

 from the observations of various experimenters, and especially from the testi- 

 mony of M. Ferussac, who says, "I have seen the terrestrial Gasteropods 

 allow their skin to be eaten by others, and, in spite of large wounds thus pro- 

 duced, show no pain." This fact has an important bearing on our general 

 views of the operations of the nervous system ; since it would seem to confirm 

 an opinion founded upon other phenomena, that the impressions which pro- 

 duce reflex actions through the nervous system do not always involve the 

 production of sensation. ( 173 182.) 



135. The nervous system of the Gasteropoda consists of at least three dis- 

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

 they supply. The anterior or cephalic ganglia are larger, in proportion to the 

 rest, than 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 extremely variable. 

 This centre, however, always bears a close relation with the gills, both in situa- 

 tion 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 functions of this ganglion are 

 subdivided .between two; of which one is still appropriated to the branchiae; 

 whilst the other is connected with the general surface of the mantle, and with 

 the respiratory passages which are prolongations of it, and hence may be 

 called the palleal 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 sepa- 

 rate 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 mus- 

 cular cavity, containing a curious rasp-like tongue, often supported upon car- 

 tilages, which serves to reduce the food ; and sometimes furnished with horny 

 maxillae. The nerves which supply these do not proceed directly from the 

 cephalic ganglia, but from a distinct centre ; from which ramifications proceed 

 along the oesophagus and stomach, and these 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 which, in the nervous system of Vertebrata, 



