COMPARATIVE ANATOMY OF THE ENCEPHALON. 



161 



Fig. 29. 



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

 Corpora Q,uadrigemina, 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 fetal or perfect state of the Human Brain. 



216. The Brain of Reptiles does not show any con- 

 siderable 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 develop- 

 ment of the Cerebellum is especially seen in the Frog 

 (Fig. 16), 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 deficiency 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 hemispheres. 

 The distinction between the tubercula quadrigemina, 

 and the parts enclosing 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 foetal 

 Mammal. 



217. This is still more evident in Birds, in whose 

 Encephalon the Tubercula duadrigemina or Optic 

 Ganglia, and the Thalami with their included ventri- 

 cle, are obviously very distinct parts. The Cerebral 

 Hemispheres attain a great increase of development, 



and arch backwards, so as partly to cover the Optic ganglia ; and these are 

 separated from one another, and thrown to either side. The Cerebellum also 

 is much increased in size, proportionably to the Medulla Oblongata and its 



Brain of Turtle ; A, olfactive 

 ganglia; B, cerebral hemi- 

 spheres; c, optic ganglia; D, 

 cerebellum. 



Fig. 30. 



thai 



Brain of Buzzard ; the olfactive ganglia are 

 concealed beneath B, the hemispheres ; c, op- 

 tic ganglia; D, cerebellum; g, pineal gland. 



The hemispheres, B, drawn to either side to 

 show the subjacent parts; c, the optic lobes; 

 D, cerebellum; thai, thalamus optic us ; e s } 

 corpus striattun. 



