NERVOUS SYSTEM. VERTEBRATA. 125 



two thirds the size of the hemispheres. The main part of 

 the hemispheres lies behind the foramen of Monro, forming 

 extensive pseudo-occipital lobes that reach backwards on 

 either side to the hinder margin of the mid-brain. The 

 anterior parts of the hemispheres are very short. The epi- 

 physis is remarkably well developed. The optic lobes and 

 cerebellum do not essentially differ from those of the Turtle. 

 The roof has been removed from the left hemisphere 

 and olfactory bulb showing the continuity of their ventricles, 

 and in the case of the former the relatively slight promi- 

 nence of the corpus striatum and the position of the choroid 

 plexus. Upon the left side, the cavities of the optic lobe 

 and cerebellum have been opened showing the thinness of 

 their walls and the forward protrusion of the choroid plexus 

 from the roof of the rhomboid fossa into the cavity of the 

 cerebellum. 



AVES. 



Bumm, Zeits.wiss. Zool., Bd. xxxviii. 1883, p. 430 ( Cerebrum). 

 Brandis, Arch. mikr. Anat., Bd. xli. 1893, p. 623 ; xliii. 



1894, p. 96;xliv. 1895, p. 534. 

 Boyce & Warrington, Phil. Trans., vol cxci B. 1899, p. 293. 



The brain is remarkably constant in form throughout the 

 class of Birds, and may be readily distinguished by its form, and 

 more particularly by the position of the optic lobes, from that of 

 any other living vertebrate. It differs from the brain of lower 

 forms mainly in its relatively greater breadth and in the 

 shortening of its base. Its most characteristic features are the 

 great development of the cerebrum, optic lobes, and cerebellum. 

 The cerebrum is usually broader than long ; its size is due almost 

 entirely to the development of the corpora striata. The hemi- 

 spheres cover the thalamus and frequently also the greater 

 part of the optic lobes, and in addition to well-marked pseudo- 

 occipital lobes usually have definite though rudimentary pseudo- 

 temporal lobes. By the forward projection of the cerebellum, 

 the optic lobes are always more or less depressed laterally towards 

 the ventral surface. The olfactory bulbs and hypothalamus 

 are weak ; the medulla is flexed as in Reptiles though not so 

 strongly. 



