82 Dr Treviranus on the 



The first class comprehends the mammalia, including man ; the 

 second, birds ; the third, reptiles ; the fourth, fish. 



The following are the most important of the other neurologi- 

 cal difrerences of these classes : 



In the mammalia, the portions from which the olfactory and 

 optic nerves take their origin, coalesce with the rest of the cere- 

 brum, no limits being distinguishable between them. In all the 

 other vertebrata, the cerebrum is distinguished into anterior and 

 posterior hemispheres, from the former of which arise the olfac- 

 tory, and from the latter the optic nerves. The posterior hemis- 

 pheres may be either separate or united together in the mesial 

 line. They are separate in birds, united in reptiles and fishes. 

 Moreover, these posterior hemispheres may be smaller or larger 

 than the anterior, — smaller in birds, reptiles, rays, and sharks ; 

 larger in other fishes. 



The mammalia possess a cerebellum, consisting of a middle 

 portion (the vermiform process) and two hemispheres, and its 

 vertical section displays an arborescent prolongation of the cord 

 (arbor vitae). In birds the hemispheres of the cerebellum are 

 merely rudimentary ; the vermiform process, however, still con- 

 tains a distinct arbor vitae. This last is entirely wanting in rep- 

 tiles and fishes, and the cerebellum consists of a simple sack, 

 formed by a thin layer of the cord. 



The mammalia alone possess the pons varolii (or great cere- 

 bral ganglion). 



In fishes, there are found at the base of the brain, behind the 

 optic tracts, two symmetrical eminences, of such extent that they 

 form the greater part of the bascj and do not much yield in size 

 to the posterior hemispheres. 



The avertebrata, in the same way, fall under two great divi- 

 sions, according to differences in the structure of their nervous 

 system. In the one, there is extended along the abdomen, in 

 two rows, or in the mesial line, a series of ganglia united to one 

 another, and to the middle of the under half of the cerebriform 

 ring by nervous filaments, and the upper part of the ring con- 

 sists of two hemispheres, immediately united together. In the 

 other division, there is no series of ganglia extending in regular 

 order the whole length of the animal, and the two lateral tube- 

 rosities of the ring are not immediately united, but by means of 



