NERVOUS SYSTEM. VERTEBRATA. 127 



form, but its apex seems to lie further forward than is 

 usually the case. The medulla is only slightly flexed. The 

 hypophysis is very large. 



D. 149. The brain of an Ostrich (StrutMo camelus) . This specimen 

 shows well the chief superficial characters of a Bird's brain : 

 great longitudinal compression of the basal parts ; minute 

 olfactory bulbs sessile on the anterior extremities of the 

 hemispheres ; strong development of the cerebrum and 

 optic lobes; depression of the latter towards the ventral 

 surface, and great size of the cerebellum. These characters 

 taken together at once distinguish this brain from that of 

 any lower vertebrate, although the development of the 

 optic lobes is equalled in many Teleosts and that of the 

 cerebellum in the higher Sharks. Each cerebral hemi- 

 sphere is short, and very broad behind, with an indentation 

 on the base in the position of the mammalian Sylvian 

 fissure, forming the anterior boundary of a small pseudo- 

 temporal lobe, and with indications upon the dorsal surface 

 of a longitudinal furrow that extends forwards to the base 

 of the olfactory bulb and forms the lateral boundary of 

 a mesial eminence. 



The hemispheres are strongly arched above, contiguous 

 in the mid-line, slightly concave below and bluntly pointed 

 in front, with the points lying close side by side and termi- 

 nating in a pair of small olfactory bulbs. The hemispheres 

 extend backwards over the roof of the thalamencephalon 

 and conceal the greater part of the optic lobes. The latter 

 are a pair of oval prominences situated on the late ro- ventral 

 aspect of the mesencephalon with their long diameter 

 directed downwards and forwards. At their anterior ends 

 they pass directly into the optic tracts, which form a complete 

 chiasma in front of the infundibulum. The chiasma has 

 been partly dissected to show the crossing of the nerve- 

 fibres. The cerebellum represents the Mammalian vermis ; 

 it is remarkably large, and forms an upstanding rounded 

 or sub-conical eminence that projects forwards between the 

 hemispheres above the roof of the mesencephalon, and 

 backwards over the rhomboid fossa. It is slightly flattened 

 from side to side and convex in front and behind, with a 



