33<^ Journal of Comtarative Neurology. 



jection into the ventricle from the base, forming the colliculus 

 or torus, with the concentric arrangement of cells character- 

 istic of the colliculi, and the rather thick tectum exhibits the 

 the same pronounced stratification of elements. Along the 

 median dorsal line the dependent ridges known as the tori 

 longitudinalis are evident, though relatively small, and seem 

 to make up for the slight development of the granular zone 

 of the tectum. 



In the drum the extensive development of the volvula, 

 and its compact plication within the ventricle serve to greatly 

 modify the tectum and other bodies. The whole organ is tilted 

 forward so that the apparent cephalic aspect really represents 

 a large part of the dorsal. The tori are therefore cut longitu- 

 dinally in transections cephalad, so that their apparent size 

 is increased. On the dorsal surface, however, the tectum is 

 forced apart, and the tori form the starting points for the 

 membranous expansion which bridges over the interval sepa- 

 rating the two halves. The colliculi are likewise thrust 

 apart and modified as to form by the same means. 



The black-horse affords an illustration of an extreme 

 modification resulting from the exceptional development of 

 the volvula. The mesencephalon in this case extends far 

 cephalad over the hemispheres, while its entire dorsal surface 

 is reduced to a membrane. The tori are thus extended in 

 the same plane as the tectum, and serve as supports for 

 the membranous roofs (Plate XIX, Fig. 4). The position 

 and structure of the corpus posterior will be discussed 

 in the histological part of this paper, as w'ill the cranial 

 nerves. 



The Cerebellum. — The present notes may be regarded as 

 supplementary to the earlier paper on the archetectonic of 

 the cerebellum. In it we attempted to show that the varia- 

 tions in structure exhibited in different groups could all be 

 reduced to a common type and their differences explained by 

 tracing the invaginations and evaginations of the walls of the 

 fourth ventricle. No better illustration of this principle 



