COMPARATIVE ANATOMY AND PHYSIOLOGY. 105 



we shall hereafter inquire, may be called, from its distribution, the stomato- 

 g as trie system. 



130. The ganglia just described may be regarded as corresponding with 

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

 ntrves is analogous. Thus the branchial ganglion obviously corresponds with 

 that portion of the Medulla Oblongata which is the centre of the respiratory 

 actions in Vertebrata. 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 connects 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 condition 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 is isolated from every other part of the nervous apparatus. The 

 cephalic ganglia must be regarded as analogous, not to any single portion of 

 the Encephalon in Vertebrata, but in some degree to the whole. We find 

 nerves of special sensation proceeding from them, certainly to eyes and an 

 auditory apparatus, perhaps also to olfactive organs, as well as others of com- 

 mon sensation, supplying the tentacula 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 Ver- 

 tebrata, as well as, perhaps, of the Cerebellum. And if we regard these 

 animals as enjoying the perceptive, reasoning and volitional faculties, in how- 

 ever low a degree, we must attribute to their cephalic ganglia some portion of 

 the attributes of the cerebral hemispheres in the highest classes. This com- 

 bination of function will not appear so extraordinary, when it is recollected 

 that all the central operations of the nervous system are performed in the 

 Tunicata by one ganglion, and in the Radiata by a series, of which each is 

 but a repetition of the rest ; and it is quite conformable to the general principle 

 of the gradual specialization of function which may be observed in ascending 

 the scale of organization. 



137. 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 distribution of some of its 

 nerves to the viscera, however, may indicate that it partly performs the func- 

 tions of that system; with which it is structurally intermixed, even in Verte- 

 brata, as the late inquiries of Miiller, Volkmann, and others (of which the 

 results will hereafter be stated), have shown. But the stomato-gastric system 

 may, perhaps, with more probability be considered as executing its offices. 

 Into the peculiar character of that system we shall be more competent to 

 inquire, when we have traced it through other classes of Invertebrata. 



138. Having thus separately considered the nervous centres of the Gastero- 

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

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



