VERTEBRATA. 



407 



fourth ventricle. The part in the optic lobes becomes con- 

 stricted into a small canal or iter leading from third to 

 fourth ventricles. 



Fig. 293. — Diagram of the Vertebrate Brain. 



(Mainly after Huxley.) 



Cerebrum. Pineal Body. 



Optic 

 Lobe. Cerebellum. Spinal Cord. 



Notochord. 



Fig. 294. — Diagram- 

 matic Median Section 

 through a vertebrate 

 Brain, showing the 

 Ventricles. 



The dorsal wall of the thala- 

 mencephalon is produced into a 

 process called the pineal body, 

 which, in some cases, shows 

 evidence of being a vestigial eye. 

 The ventral wall is also produced 

 into a process called the infundi- 

 bulum, coming into relation with 

 the pituitary body {v.i.)\ the 

 lateral walls become thickened 

 and form the optic thalami. 

 Thus the brain becomes a com- 

 plex organ consisting of a linear 

 series of specialised portions ; 

 but a further complication takes 

 place in the flexure of one part 

 upon another. In the highest 

 types (mammals) the brain is 

 twice flexed upon itself and its 

 origin from a single tube is thus disguised. 



From the brain there arise at least ten pairs of ^ cranial 

 nerves which are remarkably constant in their relationship. 

 The fore-brain gives rise to the olfactory (I.) and optic 

 (IL), the mid-brain to the oculomotor (III.) and trochlear 

 (IV.), and the hind-brain to the trigeminal (V.), abducens 



Lateral 

 Ventricle. 



Third Ventricle. 



Fourth Ventricle. 



