ENCEPHALON OF REPTILES AND BIRDS. 



283 



appears entirely changed. Hence it is not correct to assert, as is frequently 

 done, that the Brain, or any other organ, in the higher animals, passes 

 through a series of forms, which are parallel to the permanent forms of the 

 same organ in different parts of the animal scale ; since the fact is rather, that 

 the more nearly all are traced back to their first origin, the closer will their 

 conformity be found to be ; the subsequent development of each taking place 

 not only in various degrees, but in different modes or directions ; so that the 

 resemblances presented by the higher, at different epochs of their evolution, 

 to the permanent conditions of the lower, are often far from being complete.* 

 This we have seen to be the case in the present instance ; the vesicle of the 

 Corpora Quadrigemina, and that of the third Ventricle, uniting to form the 

 Optic Lobes of Fishes, whilst in the higher Vertebrata they remain distinct ; 

 so that there is no single part, with which the Optic Lobes can be properly 

 compared, either in the foetal or perfect state of the Human Brain. 



360. The Brain of REPTILES does not show any considerable advance in 

 its general structure above that of Fishes; but the Cerebral Hemispheres are 

 usually much larger in proportion to the Optic lobes ; whilst the Cerebellum 

 is smaller. The very low development of the Cerebellum is especially seen 

 in the Frog (Fig. 132), in which it is so small as not even to cover-in the 

 Fourth Ventricle; but it is common to nearly the whole group. The defi- 

 ciency in commissures still exists to a great extent. The anterior Commissure 

 in front of the third ventricle, is the only uniting band which can be distinctly 

 traced in Fishes ; and Reptiles have, in addition to this, a layer of uniting 

 fibres which may be compared to the Fornix ; but as yet, there is no vestige 

 of a true Corpus Callosum, or great transverse commissure of the hemi- 

 spheres. The distinction between the tubercula quadrigemina, and the parts 

 inclosing the third ventricle, is more obvious than in Fishes ; in fact the Optic 

 ganglia of Reptiles correspond pretty closely with the Vesicle of the tubercula 

 quadrigemina in the brain of the fcetal Mammal. 



Fig. 145. 



Fig. 146. 



Brain of Turtle ; A. olfactive 

 ganglia; B, cerebral hemi- 

 spheres: c, optic ganglia: r, 

 cerebellum. 



Brain of Buzzard ; the olfactive ganglia 

 are concealed beneath B, the hemispheres; 

 c, optic ganglia; B, cerebellum; g, pineal 

 gland 



* For a fuller examination of this interesting question, see General and Comparative Phy- 

 siology, 244. 



