xii PHYLUM CHORD ATA 517 



by a thick mass of gray matter, the middle or soft commis- 

 sure (m. co) passing across the diacoele. The floor of the 

 diencephalon is produced downwards into a mesial rounded 

 process, the tuber cinereum or infundibulum (inf) to which 

 the pituitary body is attached. 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 ante- 

 rior border of which the optic nerves are given off. Behind 

 the tuber cinereum is a rounded elevation, the corpus 

 mammillare (c. ma). 



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 (Fig. 310, 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 (Fig. 311, cb) the cerebellum exhibits a tree-like 

 pattern (arbor vita) t 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, 

 connects together the lateral parts of the cerebellum. 



The cranial nerves are similar to those of the pigeon in 

 most respects, differing in some of the particulars of their 

 arrangement and distribution. 



The rabbit, like most other vertebrates, possesses a 

 sympathetic netvous system, consisting of a series of ganglia 

 united together by commissural nerves and giving off 

 branches to the various internal organs. 



In the organs of special sense the following special 



