THE BRAIN. 



147 



connection with the mid-brain. The cerebellum usually consists 

 of a thicker median, and two fold- or wing-like lateral portions. 

 It generally overlies the sinus rhomboidalis for some distance, 

 and attains its greatest development in the Crocodilia (Fig. 121, 

 HH}. 



Judging from the casts of the cranial cavity, the brain of Dinosaurians 

 must have been very lowly organised, and much more nearly related to that 

 of Lizards than to that of Birds. The genus Stegosaurus possessed the 

 smallest brain of any terrestrial Vertebrate relatively to its size. 



Birds. 



In Birds, the hemispheres are so largely developed that they 

 overlie the anterior part of the mid-brain, bending back the 

 pineal gland, and only leaving the cerebellum uncovered (Fig. 122, 

 A and B, VH, HH}. 



Sff 



ft 



A B 



FIG. 122. BRAIN OF FIOEON. (A, from above ; B, from the side.) 



VII, cerebral hemispheres ; Z, pineal gland ; MH, optic lobes ; HH, HH 1 , cerebel- 

 lum (vermis and flocculi) ; NH, medulla oblongata ; It, spinal cord ; H, pituitary 

 body ; /, olfactory nerve ; Lol, olfactory lobe. 



The cerebellum consists of a well-developed and folded median 

 lobe, and of two lateral portions (flocculi), which vary much both in 

 form and size. Posteriorly it completely covers the fourth ventricle. 

 The two optic lobes are separated from one another and pressed 

 downwards, so as to lie on the sides of the brain in the angle between 

 the hemispheres, cerebellum, and medulla oblongata (Fig. 122, MH}, 

 and they are connected by a broad commissure. Olfactory lobes 

 are always present, but only slightly developed. The corpora striata 

 (basal portion of the cerebrum) lying within the hemispheres are 

 so largely developed that they form by far the greater part of this 

 region of the brain. 



The toothed Birds of the Cretaceous period, with Hesperornis at their head, 

 possessed a very lowly organised Reptilian-like brain, with small hemispheres 

 and large olfactory lobes. The brain of Archaeopteryx was highly developed, 

 nearly resembling that of existing Birds. 



