230 NERVOUS SYSTEM. 



pneumo-gastric, descends to the vestibule and long semi-cir- 

 cular canals. 



The spinal nerves, like those of the cranium, have gene- 

 rally a long course before they pass out through the inter- 

 vertebral foramina, and the ganglia of their posterior roots 

 are often so small, especially in osseous fishes, as to be 

 scarcely perceptible, and slight enlargements of the spinal 

 chord can sometimes be distinguished at the origins of the 

 several pairs of symmetrical nerves. The great sympathetic 

 arising from cranial nerves as high as the trigeminus, is rein- 

 forced in its course backwards by branches from the spinal 

 nerves, and forms plexuses and ganglia, as in higher classes, 

 before being distributed on the organs of the trunk ; it is 

 more distinct in the plagiostome chondropterygii than in the 

 osseous fishes, and is least developed in the cyclostome spe- 

 cies. It forms small ganglia along the sides of the vertebral 

 column, where it receives filaments from the spinal nerves, 

 and its plexuses embrace the arterial trunks before ramifying 

 on the digestive, respiratory, and generative organs. 



In the perennibranchiate amphibia, and in the larva state 

 of those which lose the gills, the spinal chord, the medulla 

 oblongata, and the cerebral parts contained within the cra- 

 nium present the same proportions and general conditions 

 which we observe as permanent characters in most of the 

 osseous fishes ; but the cerebellum is generally smaller in 

 amphibia and reptiles than in all the other vertebrata. As 

 in the lower fishes, the spinal chord in these inferior forms of 

 amphibia is prolonged, small and tapering, through the 

 greater part of the coccygeal vertebrae, without distinct en- 

 largements where the nerves usually come off to the arms 

 and to the legs. The medulla oblongata is yet broad and 

 lobed, the cerebellum in form of a very small median trans- 

 verse lobe without hemispheres, the optic lobes large, cine- 

 ritious, smooth without, hollow within, and quite exposed, 

 and the cerebral hemispheres, extended longitudinally, 

 smooth and cineritious externally, without internal ventricles, 

 and smaller than the optic lobes. The metamorphosis of the 

 caducibranchiate species changes the condition of their ner- 

 vous system from that of the lower fishes to nearly that of 

 the reptiles above them ; and these changes are effected so 

 rapidly that we can perceive a marked advancement in the 



