CENTRAL NERVOUS SYSTEM OF ARMADILLO. 289 



the ventral surface of the brain, just posterior to the infundi- 

 bulum. 



(V) Mesencephalon. The mesencephalon is that embryolog- 

 ical division of the brain which gives rise to the corpora quadri- 

 gemina and the crura cerebri. 



The corpora qnadrigemina (PI. XXV., Fig. 7, Cor. (9.) lie im- 

 mediately posterior to the optic thalami. The anterior pair of the 

 corpora quadrigemina forms an area slightly elevated above the 

 level of the optic thalami. Just posterior to them, the posterior 

 pair of the corpora quadrigemina rise to a much higher level (PI. 

 XXIV., Fig. 3, Cor. Q.} ; their most dorsal point comes up almost 

 to the level of the cerebral hemispheres. The corpora quadri- 

 gemina are wedged between the cerebellum and the cerebral 

 hemispheres. In the armadillo, they are not separated across the 

 middle, but form one body in which separation is only faintly in- 

 dicated by a shallow longitudinal furrow. The reduction in the 

 size of the anterior pair of the corpora quadrigemina is probably 

 due to the waning importance of the sense of sight. The pos- 

 terior corpora quadrigemina retain their large size, or perhaps 

 even show an increase in size, because they are not connected as 

 directly with the sense of sight. 



The cms ccrcbri arises from under the optic tract as a faint, 

 indistinct band ' of fibers, runs backwards and disappears under 

 the pons Varolii. 



(d) Metencephalon. The embryological division of meten- 

 cephalon gives rise, in the adult, to the cerebellum. 



Viewed dorsally (PI. XXIV., Fig. 4), the cerebellum presents a 

 somewhat triangular shape, where the paraflocculi (PI. XXIV., 

 Fig. 4, Par. F/.] and the posterior lobe (PL XXIV., Fig. 4, Lob. 

 P.) form the three angles. The cerebellum is much convoluted, 

 as is the case in all mammals. Its greatest diameter is transverse. 

 This large cerebellar mass hides from view the entire fourth 

 ventricle except the most posterior part (PI. XXIV., Fig. 4, Yen. 

 IV}. The cerebellum is supported and connected with the brain 

 stem by two cerebellar peduncles (PI. XXV., Fig. 7, Ped. Cer.}. 

 Anteriorly, the cerebellum is closely adapted to the contour of 

 the cerebral hemispheres. It projects forward sufficiently to hide 

 the posterior corpora quadrigemina completely. 



