84 ANATOMY OF THE RABBIT 



midbrain, is noteworthy in a mammal as lacking a cavity large 

 enough to be designated a ventricle. Instead it has a narrow 

 canal, funnel-shaped in the rabbit, 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 

 members of the anterior pair are much larger than the posterior 

 ones and correspond with the optic lobes of the frog. Its floor is 

 formed by the cerebral peduncles, the ventral surface of which is 

 composed mainly of a pair of prominent bundles of nerve fibres 

 converging from in front, having originated in the cerebral cortex 

 and passing back into the rhombencephalon. 



The parts of the mesencephalon and prosencephalon together 

 constitute the large brain, or cerebrum. 



The third primary division, the rhombencephalon, or primary 

 hindbrain, 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 conjunctiva 

 (Fig. 122). The rhombencephalon itself is divisible into two por- 

 tions, especially well defined in the mammalia, namely, the met- 

 encephalon and the myelencephalon. The former includes the 

 small brain, or cerebellum, and a ventral region, the pons, which 

 is marked by a thick transverse band of fibres on the surface. 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 membranes 

 underlying the latter, each being attached to it along a transverse 

 line near the middle of its under surface (Fig. 124). One of these, 

 the anterior medullary velum, is connected forwards with the 

 mesencephalon, while the other, the posterior medullary velum, 

 extends back from under the posterior margin of the cerebellum 

 and covers a triangular space at the caudal end of the ventricle 

 over which the cerebellum does not reach. The posterior medullary 

 velum has the same structure as the chorioid plexus of the third 

 ventricle, but is less well developed. 



