Nervous System General. 121 



and by it the Mammalian brain may be distinguished at its base 

 from that of other Vertebrata : the size of tlie Pons is directly 

 correlated with the development of the lateral lobes of the 

 Cerebellum. 



2. The division of the Brain next anterior to the Epen- 

 cephalon is termed the Mesencephalon : it includes the Nates 

 and Testes, third Yentricle, and Pineal and Pituitary glands*. 

 The Nates, or 'optic lobes,' are expansions of the processus 

 ad testes, or crura ad cerebrum (superior peduncles of An- 

 thro.) ; the layer (valvula) uniting these two crura becomes 

 thickened by transverse white fibres behind the optic lobes, 

 and these in the higher Mammals swell into a second pair of 

 tubercles, the Testes, which usually exceed the Nates in 

 breadth, but are less in length. In the groups in which the 

 eyes are relatively largest, the large proportional size of the 

 homologue of the optic lobes is significant of their important 

 relationship with the origin of the nerves of vision. These 

 * Corpora quadrigemina ' form a much smaller mass of the 

 brain than in the lower Vertebrata, are not hollow internallyf , 

 and, though in most Mammals covered by the Cerebral Hemi- 

 spheres, are in some genera more or less exposed between the 

 Cerebrum and Cerebellum. The third Yentricle has no con- 

 tinuation into the optic lobes, which it has in oviparous 

 Vertebrata ; its ascending canal to the Pineal appendage is 

 obliterated, but the descending canal to the Pituitary body is 

 retained as the Infundibulum. The third Ventricle is formed 

 by the vertical expansion of the ' iter a quarto ad tertium 

 ventriculum,' which is itself a forward continuation of the 



* The third ventricle extends upwards into the pedicle of the Conarium 

 (pineal gland) and downward into that (infundibulum) of the hypophysis 

 (pituitary gland). Owen iii. 97. 



t Van der Hoeren. Wagner says (p. 23), ' the Corpora Quadrigemina are 

 very frequently provided with an internal cavity.' In the fcetus of Man and 

 Mammals these eminences are at first single on each side, and have an internal 

 cavity, communicating with the ventricles. Q. and S. 554. 



