NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



614 



hermit crab. One ganglion, indeed, is present 

 in the Pagurus, but both its situation and office 

 alike militate against the hypothesis of its spe- 

 cial subserviency to sensation : it is developed 

 upon the end of the smooth abdominal chords, 

 and seems to have been called into existence 

 solely to regulate the actions of the muscles of 

 the claspers by which the hermit keeps firm 

 hold of the columella of its borrowed dvvel- 

 ling."*] 



On reviewing these statements of the ner- 

 vous system of the entomoid Articulata, we 

 observe that the superior ganglion of the pri- 

 mary nervous ring, or the cerebral ganglion, 

 passes through several degrees of complication 

 from the Crustacea, where it presents only 

 slight traces of a division laterally up to the 

 Insecta, as in the bee for instance, where it 

 preponderates greatly in size over the gan- 

 glions, and where the sensorial nerves arising 

 from it present distinct ganglionic enlarge- 

 ments. The anterior or cephalic primary ner- 

 vous ring itself we see to be gradually de- 

 creasing in size from the Crustacea, where it is 

 large and lengthened, to the highest Insecta, 

 the Lepidoptera, where it is much smaller, 

 and almost coalescing the superior and inferior 

 ganglions developed on it into one ganglion. 

 We observe that the number of primary ner- 

 vous rings, with their ganglia, gradually be- 

 comes more constricted from the Crustacea 

 through the Myriapoda (where they are de- 

 veloped in an undetermined length) and the 

 Arachnida, where they are much fewer, to the 

 Insecta, where, in their larva state, they ap- 

 proach the Annelides, but in their perfect state 

 we find them developed in a regular series, 

 and more concentrated in the regions of the 

 head, thorax, and abdomen. These anato- 

 mical details, together with the complicated 

 nature of the longitudinal commissures, a dis- 

 tinct system of nerves supposed to be for re- 

 spiration, and a par vagum, demonstrate a 

 close analogy between the ganglionic cord of 

 the Insecta and the spinal cord of the Verte- 

 brata, and may be considered as reasonable 

 grounds for ranking this interesting tribe of 

 animals the highest of the Articulata. 



Further details respecting the anatomy of the 

 nervous system of Insects will be found in the 

 article INSECTA. 



VEUTEBRATA. We now pass to the last 

 and highest group, the Vertebrata, where the 

 primary nervous rings of the preceding classes 

 nave become ganglions, and their commissures 

 have become primary nervous rings. In each 

 segment of their bodies there is but one gan- 

 glion developed, but that one large, and situ- 

 ated on the dorsal aspect, and each one in the 

 different segments is united to the other by 

 commissures, thus forming a large median ner- 

 vous mass, the primary characteristic of a true 

 cerebral system. This will be, of course, sub- 

 ject to infinite modifications and degrees of 

 organization. In the lower Vertebrata the gan- 

 glia and their commissures will be nearly 



* Owen's Lectures, p. 171-72. 



equally developed; in the higher ones the 

 ganglionic formation will predominate ; and 

 as these animals are characterized by this pre- 

 dominance of ganglion, its great developement 

 takes place in that part of their body which 

 is itself the most highly developed, the head, 

 and the ganglionary mass itself is called the 

 brain. On the contrary, the developement of 

 the commissures, or of the longitudinal fibres, 

 takes place in the opposing point to the head, 

 viz. the trunk, and from that results what is 

 called familiarly the spinal marrow. Again, 

 as it is the very characteristic of the nervous 

 matter to accumulate and develop itself on the 

 dorsal aspect in preference, it can easily be 

 conceived that as the ganglionic nervous matter, 

 or brain, increases in developement, so will it 

 influence the direction of the spinal marrow, 

 and, indeed, also of the whole body. For 

 instance, in the lower nervous formations the 

 brain and spinal marrow are perfectly hori- 

 zontal. As the former proceeds in develope- 

 ment, an angle, at first very acute, is formed, 

 which gradually decreases until, in the human 

 brain, the most perfect of all, it becomes a com- 

 plete right angle. Another important point 

 is, the number of ganglia and commissures that 

 may be developed". We have already observed 

 that each segment of the body of the Verte- 

 brata contains a ganglion and a primary ner- 

 vous ring: the number, therefore, of these 

 latter is unfixed and variable, and depends en- 

 tirely on the number of the segments of the 

 body, or, in other words, the length of the 

 spinal cord depends on the length of the ani- 

 mal. But, with regard to the masses of gan- 

 glionic nervous matter situated in that segment 

 of the body which is the most highly deve- 

 loped the head they ought to be developed 

 in a manner fixed and determined ; and such, 

 indeed, is the case : a division into three is 

 observable in the brains of all the Vertebrata, 

 an anterior portion or cerebrum, a posterior 

 portion or cerebellum, a median portion, the 

 tubercula quadrigemina. Thus the number of 

 ganglia forming the brain, the most highly 

 organized part of the nervous mass, is definite 

 and invariable, while the number of ganglia 

 forming the spinal cord, the least highly or- 

 ganized part of the nervous mass, is indefinite 

 and variable. The three portions of the cere- 

 bral mass, the anterior, median, and posterior, 

 will be designated by the names of frst, second, 

 and third cerebral masses ; and we shall en- 

 deavour to point out the analogies which each 

 of these portions bears in the brains of the dif- 

 ferent animals, as we ascend the scale, respecting 

 which anatomists have various opinions. 



These observations being premised, we pass 

 to the consideration of the vertebrated classes, 

 individually, in the manner proposed, com- 

 mencing with the lowest, the fishes. 



1. Pisces. In these animals the nervous 

 system presents an immense variety of forms 

 and degrees of development. Even in the 

 C'yclostomata, a division into brain and spinal 

 marrow (in the general acceptation of the terms) 

 is evident : in the former, a division into three 



