NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



Council of the College of Surgeons under the 

 judicious direction of Professor Owen. The 

 beautiful preparations of the nervous system of 

 the mussel and other animals by our friend 

 Mr. Goadby, cannot fail to excite the delight 

 and admiration of every friend to the advance- 

 ment of Physiology. One is not less asto- 

 nished at his remarkable power of manipu- 

 lation, as displayed in the dissection of the 

 soft and fragile nerves of these delicate animals, 

 than at the great ingenuity with which he has 

 displayed and perpetuated these witnesses to 

 his anatomical skill. We hope, for the sake of 

 science, that, under the liberal patronage of 

 the College Council, Mr. Goadby will be able 

 to form a large collection of dissections of the 

 Invertebrate Nervous System ; and sure we 

 are, that in nothing can the Council contribute 

 more to promote the designs of John Hunter 

 than in making his Museum the depository of 

 such a series by such an artist.] 



3. Gasteropoda. [In this order of Mollusks 

 the locomotive function is freely enjoyed, and 

 is effected in many of the genera by a powerful 

 muscular organ, which generally acts as a 

 sucker and enables the animal to adhere forci- 

 bly to the surface and draw itself on in a 

 crawling manner the well known mode of 

 progression of the common snail and slug. In 

 other genera the foot is modified according to 

 the objects to which the animals adhere, or loco- 

 motion is performed by portions of the mantle 

 adapted to act as oars or fins in swimming. 



The respiratory function, whether adapted to 

 an aquatic or a terrestrial mode of existence, 

 is much more highly developed in these animals 

 than in the preceding order. Their digestive 

 system, too, is more perfect, the accessory 

 organs being more fully developed. We per- 

 ceive, too, the unequivocal existence of a 

 visual organ. There are also special organs 

 (tentacles) for the exercise of the sense of 

 touch, and it has been supposed that the power 

 of smell and that of hearing existed, although 

 the respective seats of these senses cannot be 

 determined. 



It is well known that if the surface of a 

 snail or slug be touched ever so slightly, a con- 

 traction of the part so stimulated will imme- 

 diately take place. This is probably due to 

 the inherent irritability of the subdermic mus- 

 cular layers, or it may result from the 

 reflexion of the impression upon the motor 

 organs from some nervous centre with which 

 the nerves of the skin are in connexion. It 

 seems very unlikely that we can refer it to a 

 sensibility of the surface, for the observations 

 of Ferussac clearly imply that the terrestrial 

 Gasteropods present no signs of pain when 

 injured or wounded. We cannot, therefore, 

 agree with Professor R. Jones in assigning 

 tactile sensibility to the general cutaneous sur- 

 face of these animals, nor do we think it neces- 

 sary to regard the phenomenon in question (as 

 Dr. Carpenter suggests) as an example of 

 motion excited by a reflected impression, which 

 is not accompanied with sensation ; but rather 

 as an instance of muscular contraction, pro- 

 duced by the immediate influence of the stimu- 

 lus on the irritable fibre. 



C05 



Now this more active exercise of certain 

 functions necessarily implies a greater develop- 

 ment of the nervous system; but the same 

 general plan as that described in the Conchifers 

 prevails. The principal part of the nervous 

 apparatus is connected with the oesophagus, 

 and has communications with the other ganglia. 

 There is also a centre of locomotion, and a 

 respiratory centre.* 



The cesophageal nervous centre is developed 

 either as two small ganglia, situate on either 

 side of the oesophagus, or as a single large 

 ganglion placed on the median line and above 

 the oesophagus ; or, lastly, a single ganglion is 

 formed beneath the oesophagus. In each of 

 these varieties the common type of a nervous 

 ring or collar around the cesopliagus is pre- 

 served, however the situation of the cephalic 

 centre or the number of its ganglia may differ. 



The centre of locomotion consists of two 

 ganglia, from which nerves proceed to the foot 

 and to the mouth ; which latter, however, is 

 sometimes supplied from distinct ganglia. These 

 ganglia are connected to each other by com- 

 missural nerves and also to the cephalic centre. 



The respiratory apparatus and the viscera 

 receive nerves from a proper centre, which 

 sometimes is formed by one ganglion, some- 

 times by two separate ones, which, however, 

 have a connexion with the cephalic ganglia. 

 The branchial and the pedal ganglia are some- 

 times conjoined, and in some genera there is a 

 still further concentration, so as to form a com- 

 mon centre from which nerves are distributed 

 to all the organs. 



The following examples will serve to illus- 

 trate the principal points in the nervous system 

 of these animals : 



In Patella (limpet) there are two ganglia 

 situate on either side of the oesophagus ("A, fig. 

 339). From these ganglia the tentacles and 



. 339. 



A, cerebral ganglia. B, 

 pedal ganglion. C, bran- 

 chial ganglion, a, fila- 

 ment of communication 

 from cerebral to bran- 

 chial ganglion, b, fila- 

 ment of communication 

 from cerebral to pedal 

 ganglion. E, labial gan- 



- j glia. D, connecting band 



Nervous system of Patella between labial ganglia. 

 vulgaris ^Limpet). 



eyes are supplied with nerves, and they are 

 connected to each other by a simple nervous 

 band which passes above the oesophagus. From 

 the posterior part of each ganglion two nerves 

 pass back : the outer one terminates in the bran- 

 chialganglion,and theinneronein the pedal. The 

 apparatus for mastication in this animal being 

 complicated, it is supplied from a transverse 

 band or two ganglia, situated beneath the 

 oesophagus, and connected with the anterior or 

 cerebral ganglia. This band is connected with 

 two ganglia that supply the lips, called labial 



* See article GASTEROPODA, p. 394, vol. ii. f 

 and Dr. Carpenter's Lectures on the Nervous Sys- 

 tem, Lond. Med. Gazette for 1841. 



