102 FUNCTIONS OF THE NERVOUS SYSTEJVI. 



Hydra and the most perfect Vertebrated animal. In the latter, however, an- 

 other set of muscles are superadded to these, for the purpose of preparing the 

 aliment by mastication for the operation of the stomach, and of bringing it 

 within reach of the pharyngeal constriction. But, it has been urged, the inac- 

 tivity of the tentacula when the Hydra is gorged with food, proves that they 

 are excited to action by the will of the animal. This inference, however, may 

 be easily disproved. The muscles of deglutition in Man are not called into 

 action with nearly the same readiness and energy, w T hen the stomach is dis- 

 tended, as when it is empty ; a fact of which any one may convince himself, 

 by observing the relative facility of swallowing at the commencement and the 

 termination of a full meal. No one will assert that this variation is an effect 

 of the will ; indeed, it is often opposed to it ; being one of those beautiful 

 adaptations, by which the welfare of the economy is provided for, but which 

 the indulgence of the sensual appetites opposes. Most of the movements of 

 this animal, and of others of the class, appear to be equally the result of ex- 

 ternal stimuli with that already described ; and it is only in a few instances, 

 principally those of absolute locomotion or change of place, that any evidence 

 of voluntary action can be discerned. It may be occasionally remarked, how- 

 ever, that one or more of the tentacula are retracted or extended, without the 

 slightest appreciable change in any of those external circumstances which 

 seem ordinarily to affect the motions of the animal ; and this action we can 

 scarcely regard as otherwise than voluntary. 



131. Thus in the Nervous System of Radiated Animals, we have an instance 

 of that community of function which is so remarkable in the organism of the 

 lower tribes, when contrasted with the separation which is perceptible in those 

 at the opposite extremity of the scale. The visceral nerves of the Asterias are 

 not isolated at their central terminations from those which are connected with 

 the sensorial and locomotive functions : nor are those which minister to the 

 instinctive actions separable from those which convey the influence of the 

 will. Every segment of the body appears equal in its character and endow- 

 ments to the remainder ; each has a ganglion appropriated to it ; and, as the 

 ganglia, like the segments, are all alike, neither of them can be regarded as 

 having any presiding character. 



132. From the Radiated we now pass to the MOLLUSCOUS glasses, the gene- 

 ral character of which, as a natural group, is the remarkable predominance of 

 the Nutritive system over that of Animal life. In fact, although the organs 

 which minister to their vegetative functions attain a very high degree of deve- 

 lopment, the animal powers of sensation and locomotion are, in general, so 

 feebly manifested, as to show that they are entirely subservient to the exercise 

 of tMe former. There is not in the Mollusca, as in theRadiata, any repetition 

 of parts around a common centre ; and we do not, therefore, meet in them with 

 a number of ganglia nearly or altogether alike in endowments. In some of 

 the higher species, there is a conformity between the two sides of the body, 

 or a lateral symmetry : which involves a subdivision of some of the ganglia, 

 that are single in the inferior tribes, into two masses, which always remain in 

 connection with each other. With this exception, it may be observed, that all 

 the ganglia, to the number of four or five, which we meet with in the higher 

 Mollusca, appear to have distinct functions ; as may be determined by tracing 

 the .distribution of their nerves. Thus we find a pair of cephalic ganglia, 

 situated above the oesophagus, connected with the organs of special sensation, 

 and sending motor nerves (as we shall see reason to believe) to all parts of the 

 body. This is obviously analogous to the brain of Vertebrata. Below the 

 oesophagus there is generally a small ganglion, connected with the apparatus 

 of deglutition, which may be called the stomato-gastric ganglion. In connec- 

 tion with the gills we have always one ganglion, sometimes a pair, which may 



