NERVOUS SYSTEM OF THE VERTEBKATA. 425 



conveniently spoken of as the cerebellum (figs. 247 and 248, cb) 

 and medulla oblongata. The floor of these regions is quite 

 continuous and is also prolonged without any break into the 

 floor of the mid-brain. 



The posterior section of the hind-brain, which forms the 

 medulla, undergoes changes of a somewhat complicated charac- 

 ter. In the first place its roof becomes in front very much 

 extended and thinned out. At the raphe, where the two lateral 

 halves of the brain originally united, a separation, as it were, 

 takes place, and the two sides of the brain become pushed apart, 

 remaining united by only a very thin layer of nervous matter, 

 consisting of a single row of flattened cells (fig. 249). As a 

 result of this peculiar growth in the brain, the roots of the nerves 

 of the two sides, which were originally in contact at the dorsal 

 summit of the brain, become carried away from one another, and 

 appear to arise at the sides of the brain. 



The thin roof of the fourth ventricle is triangular, or, in 

 Mammalia, somewhat rhomboidal in shape. The apex of the 

 triangle is directed backwards. 



At a later period the blood-vessels of the pia mater form a 

 rich plexus over the anterior part of the thin roof of the medulla, 

 which becomes at the same time somewhat folded. The whole 

 structure is known as the tela vasculosa, or choroid plexus 

 of the fourth ventricle (fig. 250, chd 4). The floor of the 

 whole hind-brain becomes thickened, and there very soon 

 appears on its outer surface a layer of non-medullated nerve- 

 fibres, similar to those which first appear on the spinal cord. 

 They are continuous with a similar layer of fibres on the floor 

 of the mid-brain, where they constitute the crura cerebri. On 

 the ventral floor of the medulla is a shallow continuation of the 

 anterior fissure of the spinal cord. 



In Elasmobranchii and many Teleostei the restiform tracts are well 

 developed, and are anteriorly continued into the cerebellum, of which they 

 form the peduncles. Near their junction with the cerebellum they form 

 prominent bodies, which are regarded by Miklucho-Maclay as representing 

 the true cerebellum of Elasmobranchii. 



In Elasmobranchii a dorsal pair of ridges projects into the cavity of the 

 fourth ventricle, corresponding apparently with the fasciculi teretes of the 

 Mammalia. 



In Mammalia there develop, subsequently to the longitudinal fibres 



