CHAP. X.] NERVOUS SYSTEM IN INVERTEBRATA. 247 



also in the light of a ganglion compounded of smaller ones, which 

 have been, as it were,, fused together. And certain anatomical indi- 

 cations in the lower animals, as well as in man, favour this view; 

 thus, in the common gurnard (trigla lyra), there is a series of gan- 

 gliform swellings situate on the posterior surface of the cervical 

 portion of the cord at its upper part, from which large nerves pass 

 off to the feelers ; and in all animals the cord exhibits a distinct 

 enlargement, at each segment with which large nerves are connected, 

 or a contraction, if the nerves be of small size and of comparatively 

 little physiological importance. 



The cerebro-spinal axis, with the nerves pertaining to it, consti- 

 tutes the greatest portion of the nervous system of the human sub- 

 ject and of the vertebrate animals. The sympathetic system, how- 

 ever, is connected with a large number of those parts on which the 

 principal organic functions depend. This portion of the nervous 

 system always bears a direct relation in point of development to 

 that of the cerebro-spinal portion, with every part of which it is 

 very intimately associated. If we except the olfactory, optic, and 

 auditory nerves, there is no nerve with the origin of -which it does 

 not form a connexion. Its segments remain separate, as distinct 

 ganglia, connected, however, by intercommunicating cords passing 

 from one to the other, by which the continuity of the chain on 

 either side of the vertebral column is maintained. In mammals and 

 birds the sympathetic is fully developed ; but in some reptiles and 

 fishes it is partly deficient, and its anterior part, which is wanting, 

 is supplied by the vagus nerve. In the cyclostomatous fishes, as 

 the lamprey, it is wanting altogether ; and the vagus seems to supply 

 its place. In no animal is it so fully developed as in man. 



Arrangement of the Nervous System in Invertelrata. 



It is foreign to the purpose of this work to enter into details of comparative 

 anatomy. The following paragraphs are merely intended to call the reader's 

 attention to the general plan of the nervous system in the Invertebrata. The 

 arrangement adopted is that suggested by Professor Owen. 



The Invertebrate animals may be classed in three groups, according to the 

 prevailing type of arrangement of the nervous system. 1. The first, or Nema- 

 toneurose, exhibits no other trace of nervous system than is to be found in 

 simple threads or filaments. In the asterias, one thread surrounds the mouth, 

 and others pass from it to the rays ; and in the strongylus gigas, a slender 

 nervous ring surrounds the upper part of the gullet, and from it a single 

 thread is continued along the ventral surface to the opposite extremity, where 

 another nervous loop is found surrounding the anus. No distinct evidence of 

 the existence of ganglia in animals included in this group has been obtained. 

 It would be premature, however, to suppose that the absence of gangliforin 

 Swellings implies that of vesicular nervous matter. 



2. The second group of animals is designated Heterogangliate, from the 



