CHAPTEB VIII. 



THE BRAIN OF FISHES AND OF AMPHIBIA. 



IN all Vertebrates the relation of the principal nervous 

 ganglia to the commencement of the alimentary canal is 

 different from that existing among the Invertebrates. 

 We no longer find, as in the Mollusk, the Worm, or the 

 Insect a ring of nerve matter encircling the oesophagus. 

 The parts which in FISHES answer to the supra- and sub- 

 ossophageal ganglia lie altogether above the oesophagus, 

 and they are, moreover, directly continuous with one 

 another, instead of being connected by long or short com- 

 missures. 



In Fishes, as well as in other Vertebrates, all the parts 

 constituting the Brain, as well as the Medulla Oblongata, 

 are enclosed within a distinct ' skull' or ' cranium,' while 

 within this they are again surrounded by two membranes 

 one of which, and the thicker of the two, lines the 

 inner surface of the cranium ; while the other, which is 

 delicate and transparent, immediately envelops the great 

 nerve centres. The Spinal Cord, which is directly con- 

 tinuous with the Medulla, is also lodged in a bony case 

 known as the ' spinal canal ' ; and this is formed by the 

 contiguous posterior arches of the several vertebrae com- 

 posing the spine or vertebral column. 



Among the Invertebrata, it is the nervous system 



