74 Anatomy of the Rabbit. 



Its cavity is a narrow canal, the cerebral aqueduct, leading from 

 the third ventricle backward to the fourth ventricle, or cavity of the 

 rhombencephalon. Externally, its roof is differentiated into four 

 rounded elevations, the corpora quadrigemina, of which the 

 anterior pair are much larger than the posterior ones. Its floor 

 is chiefly formed by a pair of divergent cords, the cerebral pe- 

 duncles. 



The parts of the mesencephalon and prosencephalon together 

 constitute the large brain, or cerebrum. 



The third primary division, the rhombencephalon, or primary 

 hind-brain, is a greatly elaborated portion from which arise the 

 majority of the cranial nerves. The constricted area joining it with 

 the mesencephalon is known as the isthmus rhombencephali. 

 It includes the anterior medullary velum and brachia con- 

 junctiva (Fig. 86). The rhombencephalon itself is divisible into 

 two portions, especially well defined in the mamalia, namely, the 

 metencephalon, or hind-brain, and the myelencephalon, or 

 after-brain. The former includes the small brain, or cere- 

 bellum, and a ventral structure of a commissural nature, the pons. 

 The myelencephalon is a transitional portion connecting the brain 

 with the spinal cord. The cavity of the rhombencephalon is the 

 fourth ventricle. It is a peculiarly shaped space, the floor and 

 lateral walls of which are very greatly thickened, while the roof is 

 for the most part thin. The roof appears at first sight to be formed 

 largely by the cerebellum, but is in reality formed by two mem- 

 branes underlying the latter. One of these, the anterior medul- 

 lary velum, is connected forwards with the mesencephalon, while 

 the other, the posterior medullary velum, covers a triangular 

 space over which the posterior margin of the cerebellum does not 

 extend^? The posterior medullary velum has the same structure 

 as the chorioid plexus of the third ventricle, but is much more 

 poorly developed. 



' Apart from its principal divisions, which, as indicated above, 

 are more or less common to all vertebrates, the brain depends for 

 its external form on the elaboration of certain parts in comparison 

 with others. In the mammalia the cerebral hemispheres and the 

 cerebellum are the chief form-determinants, although the pons and 

 the corpora quadrigemina may also be considered in this connection. 



