118 FUNCTIONS OF THE NERVOUS SYSTEM. 



to prove it definitely ; since all analogy leads to an opposite conclusion. We 

 may regard the capability of separating a particular' secretion from the blood, 

 as a peculiar property inherent in the glandular membrane, just as contractility 

 is the inherent property of muscular fibre. As the peculiar arrangement of 

 the excitable and contractile tissues in Animals requires a nervous system to 

 act as a conductor between them, and to blend their actions, so may the com- 

 plicated organic functions of Animals require to be harmonized and kept in 

 sympathy with each other, by some mode of communication more direct and 

 certain than that afforded by the circulating system, which is their bond of 

 union in Plants. We have seen, in the foregoing sketch, that the visceral 

 system does not exist in a distinct form in the lower classes of Invertebrated 

 animals ; and also that the nervous system of these classes cannot, as a whole, 

 be compared with it, although it may be regarded as containing some rudi- 

 ments of it. As the divisions of this system become more evident, however, 

 and the organic functions more complicated, some appearance of a separate 

 Sympathetic system presents itself; but this is never so distinct as in Verte- 

 brata. Hence it may fairly be inferred that, as the Sympathetic system is 

 not developed in proportion to the predominant activity of the functions of 

 organic life (which is so remarkable in the Mollusca when contrasted with the 

 Articulata), but in proportion to the development of the higher divisions of the 

 nervous system, its office is not to contribute to these functions any thing 

 essential to tljeir performance ; but rather to exercise that general control over 

 them, which becomes the more necessary as they become more independent of 

 one another; and to bring them into relation with the system of Animal life. 



VII. Nervous System of Vertebrata. 



157. When we direct our attention to the Nervous system of the Verte- 

 brated classes, we are immediately struck by two remarkable differences which 

 its condition presents, from that under which we have seen it to exist in the 

 Invertebrata. In the latter it has seemed but a mere appendage to the rest of 

 the organism, a mechanism superadded for the purpose of bringing its various 

 parts into more advantageous relation. On the other hand, in the Vertebrata 

 the whole structure appears subservient to it, and designed but to carry its 

 purposes into operation. Again, in the Invertebrata, we do not find any special 

 adaptation of the organs of support, for the protection of the Nervous System. 

 It is either enclosed, with the other soft parts of the body, in one general hard 

 tegumentary envelop, as in the Echinodermata and Articulata, or it receives 

 a still more imperfect protection, as in the Mollusca. In the latter, the naked 

 species are destitute of any means of passive resistance, and the Nervous Sys- 

 tem shares the general exposed condition of the whole body ; and it is not a 

 little remarkable that, in the testaceous kinds, the portion of the body contain- 

 ing the nervous centres should be protruded beyond the shell, whilst the prin- 

 cipal viscera are retained within it. Now, in the Vertebrata, we find a special 

 and complex bony apparatus, adapted in the most perfect manner for the pro- 

 tection of the Nervous system ; and it is, in fact, the possession of a jointed 

 spinal column, and of its cranial expansion, which best characterizes the group. 



158. When we look more particularly at the Nervous Centres themselves, 

 we perceive that they combine the general characters of those of the Articu- 

 lata with those of the Mollusca. In the former, the power of active locomotion 

 seems the chief object to be attained ; and the predominant part of the appa- 

 ratus is evidently the series of ganglia connected with the locomotive organs'. 

 The sensory ganglia appear subservient to these both in size and func- 

 tion. On the other hand, in the Mollusca, the sensory ganglia predominate ; 

 and under their function, which is to direct these walking stomachs to their 



