NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



613 



matter of which they are composed appears 

 more dense and opaque in its texture.* 



Fig. 349. 



Ventral nenmis cord of Mormo mama (perfect state), 

 magnified to about twice the natural size. 



a, infra-uesophageal ganglion. b, large thoracic 

 ganglia, c, c, c, small abdominal ganglia. 



In Insects we observe a remarkable corre- 

 spondence between the disposition of the ner- 

 vous system and the form of the animal, and 

 this is conspicuous not only in the adult but 

 also in the larva state. Indeed the changes 

 which take place in the arrangement of the 

 nervous system as the creature passes from its 

 immature to its mature condition, are sufficient 

 to indicate that the same law which influences 

 that alteration of form, promotes the adaptation 

 of the nervous system to it ; and yet, notwith- 

 standing its apparent complication, the nervous 

 system of insects has the same physiological 

 signification as that of Mollusks. A cephalic 

 ganglion, with which are united the nerves of 

 the organs of sense, is so connected with the 

 remaining ganglions, that its influence can ex- 

 tend throughout the whole system. Each seg- 

 ment is provided with a ganglion, which has 

 no _ power beyond the limits of the segment, 

 and which cannot act consentaneously with its 

 fellows, except under the direction of the ce- 

 phalic ganglion. The pedal ganglion of Mol- 

 lusca is in insects represented by the aggregate 

 of these ganglia of the segments, which are 

 also doubtless the centres of the respiratory 

 actions. And those nerves which, arising from 

 the cephalic ganglion, are distributed to the 

 digestive organs, the stomato-gastric nerves, 

 are analogous to the sympathetic or to the 

 vagus. 



[It has been supposed by some anatomists 

 that a distinct isolation of motor and sensitive 

 function occurs in the ganglionic and non-gan- 

 ghonic cords of the abdominal nervous chain of 



* This is, however, but a rough sketch of the 

 interesting changes that take place in the nervous 

 system during the progress of the insect from its 

 larva to its perfect state. Those who are interested 

 in the matter, I beg leave to refer to Mr. Newport's 

 highly valuable paper, where, as I have before ob- 

 served, the changes of the Sphinx ligustri are de- 

 tailed with minute accuracy. See also iNSECTA. 



insects, as well as of other Articulata. (See 

 fig. 411, art. INSECTA, vol. ii. p. 952.) But 

 there are many objections to this hypothesis, 

 which, indeed, must be regarded as quite un- 

 tenable. It has been founded upon the anato- 

 mical fact, which is true as regards the verte- 

 brata, that sensitive nerves have ganglions while 

 the motor ones are devoid of them. But it is 

 going too far to compare nerves and centres, 

 and to argue from the nerves of vertebrata re- 

 specting the centres of Invertebrata. More- 

 over, as Prof. Owen remarks, the presence of 

 ganglia on the sensitive roots of spinal nerves 

 is not their constant character. This hypo- 

 thesis also received some support from a doc- 

 trine which was countenanced by Bell, namely, 

 that the columns of the spinal cord of verte- 

 brata corresponded in function with that of the 

 roots of the nerves, the anterior columns being 

 motor as the anterior roots were, and the poste- 

 rior columns and roots being sensitive. But this 

 doctrine is utterly without foundation, as will 

 be shewn in a subsequent part of this article. 

 Prof. Owen adduces an important fact respecting 

 two nearly allied Crustacea, which further inva- 

 lidates the supposed difference of function of the 

 ganglionic and non-ganglionic columns. " In 

 the lobster (Astucus) and in the hermit-crab 

 (Pagurus) we have two opposite conditions 

 of a large and important part of the trunk. In 

 the lobster the abdomen or tail is encased in a 

 series of calcareous rings forming a hard and 

 insensible chain armour, but as it is almost the 

 exclusive organ by which the animal swims, it 

 enjoys considerable motor power, a large por- 

 tion of the muscular system being devoted to it. 

 In the hermit-crab, on the other hand, the mus- 

 cular system is almost abrogated in the long 

 abdomen, for this in fact takes no share in the 

 locomotive functions of the body : it is occu- 

 pied by part of the alimentary canal and by 

 glandular organs : the sensibility of the external 

 integument is not impaired or destroyed by the 

 deposition of calcareous particles in its tissue, 

 but it retains the necessary faculty of testing 

 the smooth and unirritating condition of the 

 inner surface of the deserted shell which the 

 animal chooses for its abode: minute acetabula 

 are developed in groups upon this sensitive 

 integument, to which also delicate ciliated pro- 

 cesses are attached. The muscular system is 

 reduced to a few minute fasciculi of fibres regu- 

 lating the action of the small terminal claspers. 

 Now," adds Professor Owen, " if, as has been 

 conjectured, the ganglionic enlargements of the 

 abdominal cords monopolize the sensorial func- 

 tions, arid the non-ganglionic tracts the motor 

 powers, we ought to find the nerves, which 

 supply the muscles of tail constructed almost 

 exclusively for locomotion, to be derived from 

 non-ganglionic columns; whilst in the tail, 

 which is almost as exclusively sensitive, the 

 ganglions ought to have been large and nume- 

 rous for the supply of nerves to the integument. 

 The contrary, however, is the fact; six well- 

 developed ganglions distribute nerves to the 

 muscular fibres of the lobster's tail; non-gan- 

 glionic columns supply the sensitive tail of the 



