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 rhombencephaili. 
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. 
<. > Ph oe 
