AMPHIBIA. 45 



mammalia with respect to the lateral ventricles, but behind these and out of the 

 cavities, and their situation is even more remarkable in the frog. In having posterior 

 as well as anterior commissures, the anterior alone existing in some fishes. The 

 ventricle in the optic lobe is very similar ; but in the floor of this, the longitudinal 

 bands extending towards the calamus scriptorius are more prominent. It differs in 

 the very dense membrane covering it, especially in the turtle, in not having so much 

 space in the skull, and not being placed in a fine reticulated membrane containing 

 fluid. In having a pineal gland as well as pituitary one. The cerebellum differs from 

 that of fishes in its roundness, and thinness of its parietes, and the greater capacity 

 of its ventricle. 



In the snake, the brain is nearly the same as in the turtle ; and there is not 

 much difference in the frog, except that the thalami appear externally. In the snake 

 and frog the cerebellum is so small, as hardly to bear a comparison with that of the 

 turtle or fishes. 



In amphibia the nerves are firmer than in fishes, and acquire much magnitude 

 from the addition of neurilema. In the snake, those about the throat are not only 

 tortuous, but in a considerable degree elastic, and thus allow of being sufficiently 

 extended in deglutition. They are large in comparison with the whole brain, but 

 accord with the size of the oblong and spinal medulla, as in fishes. In the turtle and 

 snake, the olfactory nerves have not distinct ganglia : after being joined by the first 

 trunk of the fifth, they are distributed in coarse branches on the Schneiderian mem- 

 brane, and appear similar ; but it may be remarked, that their structure corresponds 

 very much with that of the splanchnic plexuses, which have not ganglia intermixed 

 with, or giving immediate origin to, them. The origin of the optic nerves is from the 

 optic lobes and thalami, and not from the optic lobes and mammillary eminences, as 

 in fishes. In the other nerves of the orbit there is not any particular change, except 

 that the sixth is larger, and supplies the additional muscles, which do not exist in 

 fishes ; and in the turtle the ciliary nerves proceed from a junction of branches from 

 the third and fifth nerves, but this has not the form of a ganglion. The fifth has not 

 a very distinct smaller portion for joining the third trunk, but there is a greater 



