NERVOUS SYSTEM OF HIGHER MOLLUSCA. 253 



which in the nervous system of Vertebrata we shall hereafter inquire, may be 

 called, from its distribution, the stomato- gastric system. 



320. The ganglia first described may be regarded as corresponding with 

 those parts of the nervous centres in the Vertebrata, the distribution of whose 

 nerves is analogous. Thus the branchial ganglion obviously corresponds 

 with that portion of the Medulla Oblongata which is the centre of their respi- 

 ratory actions ; and the pedal ganglion is analogous to that division of the 

 Spinal Cord from which the nerves of the anterior or posterior extremities 

 pass off. It is well known that such portions of the spinal cord may be com- 

 pletely isolated, without destroying the functions to which they minister. 

 Thus, the brain and lower part of the spinal cord may be removed, that 

 portion only of the cerebro-spinal axis being left, which is connected with the 

 principal respiratory nerves, in fact the respiratory ganglion, and yet the 

 animal may continue to exist for some time. It is then reduced to a condi- 

 tion similar to that of the Tunicata; whose single ganglion, though combining 

 in some degree the functions of those which exist separately in the higher 

 tribes, has evidently the regulation of the respiratory movements for its chief 

 object. In the same manner, the integrity of the segment of the cord, with 

 which the nerves of the extremities are connected, will enable them to execute 

 those movements of a reflex character, which depend upon its power as their 

 centre; even though it be isolated from every other part of the nervous ap- 

 paratus. The cephalic ganglia must be regarded as chiefly analogous to those 

 portions of the Encephalon of Vertebrata, which are immediately connected 

 with the nerves of sense. We find nerves of special sensation proceeding 

 from them, certainly to eyes and an auditory apparatus, perhaps als%> to 

 olfactive organs ; as well as others of common sensation, supplying the ten- 

 tacula and mouth. Hence we must admit, that they perform the functions of 

 the optic ganglia of Vertebrata, and perhaps also of the olfactory lobes ; as 

 well as of the portion of the medulla oblongata, in which the sensory portion 

 of the fifth pair terminates. Moreover, they certainly give origin also to 

 motor nerves ; and must thus perform the functions of the Medulla Oblongata, 

 from which the corresponding nerves arise in Vertebrata ; as well as, perhaps, 

 of the Cerebellum. It is obvious that the portion of the Nervous system of 

 the Gasteropod Mollusca, into the analogies of which we have thus inquired, 

 cannot in the least be compared as a whole with the Sympathetic system of 

 the Vertebrata, which it was formerly imagined to resemble. The distribu- 

 tion of some of its nerves to the viscera, however, may indicate that it partly 

 performs the functions of that system; with which it is structurally inter- 

 mixed, even in Vertebrata. But the stomato-gastric system may, perhaps, 

 with more probability, be considered as executing its offices. Into the pecu- 

 liar character of that system we shall be more competent to inquire when we 

 have traced it through other classes of Invertebrata. 



321. Having thus separately considered the nervous centres of the Gaste- 

 ropoda, and determined their special functions by their structural relations, we 

 shall inquire into the mode in which these functions are combined, so as to 

 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 Articulated and Vertebrated animals. If we examine the mode in 

 which the different ganglia are united by connecting trunks, we are led to per- 

 ceive 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. Frequently a 

 communication with one another appears to exist, where there is really none. 

 Thus, in the Jlplysia,z cord passes from the branchial ganglion (Fig. 125, D), 



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