OF THE NERVOUS SYSTEM. 461 



mi and these corpora. The cartilaginous fishes have no brain. 

 (Desmoulins.) In the osseous fishes it is formed by the optic tha- 

 lamus alone, which is solid. (Desmoulins.) In reptiles and birds, 

 it is formed by the same body, which is hollow, and bears 

 some resemblance to the hemispheres of the mammifera; but 

 these hemispheres do not cover the tubercula quadrigemina, 

 and have as yet neither lobes, nor circumvolutions, nor corpus 

 callosum. The brain of the mammifera, which is formed by 

 a recurved medullary membrane, whose fibres come from the 

 corpora pyramidalia, optic thalami and corpora striata, gradu- 

 ally approaches to that of man, presenting various degrees 

 of organization. The rodentia and cheiroptera occupy the 

 lowest rank in this respect. Their hemispheres do not en- 

 tirely cover the tubercula. They have only a superficial fis- 

 sura sylvii, a few slight grooves, and no circumvolutions. In 

 the carnivora, the ruminantia, the hog and the horse, the he- 

 mispheres, which are much more voluminous and prominent, 

 cover a part of the cerebellum. Th'ey have circumvolutions 

 and anfractuosities, but are still destitute of posterior lobes. 

 In the quadrumana, the hemispheres cover the cerebellum, 

 but the posterior lobe is still destitute of circumvolutions. 



The corpus callosum, which is formed by the reflection to- 

 wards the median line of the fibres of the peduncles spread 

 out in the hemispheres, do not exist in the ovipara. In the 

 mammifera its extent is proportionate to that of the hemi- 

 spheres. It is accordingly very small in the rodentia. 



The lateral ventricles, which are formed by the replication 

 of the nervous membrane of the hemispheres, are propor- 

 tionate to the extent of the latter. 



The fornix does not exist in fishes. We find the first traces 

 of its pillars in reptiles, and still more distinctly in birds. In 

 all the mammifera the pillars are united to form the fornix. 

 In them there occur, moreover, the septum lucidum and its 

 ventricle. These parts are proportionate to the extent of the 

 hemispheres. 



The cornu Ammonis exists only in the brain of the mam- 

 mifera. The unciform eminence exists in none of the animals, 

 excepting perhaps the quadrumana. 



