5io MANUAL OF ZOOLOGY SECT. 



The diaccele is a laterally compressed cavity. From 

 the posterior part of its roof arise the peduncles of the 

 pineal body, and just behind their point of origin is 

 the posterior commissure^ a delicate transverse band of 

 fibres connecting together the posterior parts of the 

 optic thalami. The latter are large masses of mixed 

 gray and white matter forming the lateral portions of the 

 diencephalon ; they are connected together by a thick mass 

 of gray matter, the middle or soft commissure passing across 

 the diaccele. The floor of the diencephalon is produced 

 downwards into a mesial rounded process, the tuber cinereum 

 or infundibulum, to which the pituitary body (hp.) is at- 

 tached. In front of 'this, on the ventral aspect of the brain, 

 is a thick curved transverse band of nerve fibres, the united 

 optic tracts, from the anterior border of which the optic 

 nerves are given off. Behind the tuber cinererum is a 

 rounded elevation, the corpus mammiUare. 



In the mid-brain the dorsal part is remarkable for the 

 fact that each optic lobe is divided into two by a transverse 

 furrow, so that two pairs of lobes, the corpora quadrigemina, 

 are produced. On the ventral region of the mid-brain the 

 crura cerebri are far more prominent than in the lower 

 groups. In the hind-brain the cerebellum (cb'. cb".) is very 

 large : it consists of a central lobe or vermis and two lateral 

 lobes divided by very numerous fissures or sulci into a large 

 number of small convolutions ; each lateral lobe bears an 

 irregularly shaped prominence, the flocculus. On section 

 the cerebellum exhibits a tree-like pattern (arbor vitce) 

 brought about by the arrangement of the white and gray 

 matter. On the ventral aspect of the hind-brain a flat 

 band of transverse fibres the pons Varolii (p. v.) connect 

 together the lateral parts of the cerebellum. 



The cranial nerves are similar to those of the Pigeon in 



