426 THE HIND-BRAIN. 



already spoken of, first the olivary bodies of the ventral side of the medulla, 

 and at a still later period the pyramids. The fasciculi teretes in the cavity of 

 the fourth ventricle are developed shortly before the pyramids. 



When the hind-brain becomes divided into two regions the 

 roof of the anterior part does not become thinned out like that 

 of the posterior, but on the contrary, becomes somewhat thick- 

 ened and forms a band-like structure roofing over the anterior 

 part of the fourth ventricle (fig. 247 and fig. 253, cb). 



This is a rudiment of the cerebellum, and in all Craniate Ver- 

 tebrates it at first presents this simple structure and insig- 

 nificant size. In Cyclostomata, Amphibia and many Reptilia 

 this condition is permanent. In Elasmobranchii, on the other 

 hand, the cerebellum assumes in the course of development a 

 greater and greater prominence (fig. 248, cb), and eventually 

 overlaps both the optic lobes in front and the medulla behind. 

 In the later embryonic stages it exhibits in surface-views the 

 appearance of a median constriction, and the portion of the 

 ventricle contained in it is prolonged into two lateral out- 

 growths. 



Miklucho-Maclay, from his observations on the brains of adult Elasmo- 

 branchii, was led to regard what is here called the cerebellum as identical 

 with the mid-brain, and the true mid-brain as part of the thalamencephalon. 

 Miklucho-Maclay was no doubt misled by the large size of the cerebellum, 

 but, as we have seen, this body does not begin to be conspicuous till late in 

 embryonic life. 



The mid-brain and thalamencephalon (according to the ordinary inter- 

 pretations) have in the embryo of Elasmobranchs exactly the same relations 

 as in the embryos of other Vertebrates ; so that the embryological evidence 

 appears to me to be conclusive against Miklucho-Maclay's view. 



In Birds the cerebellum attains a very considerable develop- 

 ment (fig. 250, cbl\ consisting of a folded central lobe with an 

 arbor vitae, into which the fourth ventricle is prolonged. There 

 are two small lateral lobes, apparently equivalent to the flocculi. 

 Anteriorly the cerebellum is connected with the roof of the mid- 

 brain by a delicate membrane, the velum medullas anterius, 

 or valve of Vieussens (fig. 250, vtna). The pons Varolii of 

 Mammalia is represented by a small number of transverse 

 fibres on the floor of the hind-brain immediately below the 

 cerebellum. 



In Mammalia the cerebellum attains a still greater develop- 



