276 ANATOMY OF THE RABBIT. 
(c) The third cranial, or oculomotor nerve (n. oculomotorius), 
arises from the ventral surface of the cerebral peduncle. 
IN THE RHOMBENCEPHALON: 
(a) The cerebellum forms an arch over the posterior portion 
of the brain, and is supported by stout pillars from its sides. 
In addition to the ridges of its surface the whole structure is 
divided into several lobes, including the median portion or 
vermis, the lateral hemispheres, and the stalked bodies, or 
paraflocculi, arising from the 
hemispheres. 
(b) The anterior medullary 
velum (velum medullare an- 
terius) is the thin membrane 
underlying the anterior portion 
of the cerebellum and connect- 
ing the latter with the inferior 
colliculi. It forms a small an- 
terior portion of the roof of the 
fourth ventricle. 
(c) The fourth cranial, or troch- 
lear nerve (n. trochlearis), 
arises from the anterior medul- 
lary velum. The first portion ot 
the nerve is usually to be found 
on the lateral surface of the 
cerebral peduncle. 

Fic. 85. The rhombencephalon. f 
Ventral view (the cerebellum not (d) The posterior medullary 
figured). y 
c.t., trapezoid body;  f.c., foramen velum (velum medullare pos- 
caecum; fi.c., cervical flexure; f.m.a., . : ae 
anterior median fissure; p., pons; terius) underlies the posterior 
p.c., cerebral peduncle (mesence- : 
pherOn) yD icn De ae: margin of the cerebellum, and 
1 oculomotor nerve; IV, troch- : 
Heat ay bos etalon OF gERERIEG, extends backward over the tri- 
minus; V2, portio minor; VI, ab- b h 
ducens; VII, facial; VIII, acoustic. Ss 
IX-XI, glossopharyngeal, vagus, and angular space enc osed yee e 
Spinal a ccesseLy tOUD ss Cat abo, walls of the fourth ventricle. 
glossal; CI, first cervical! spinal. Sie 
The structure forms a chorioid 
plexus, similar in character to that of the third ventricle but 
much less extensive. It is commonly torn away in the 
preparation of the braia, in which case the interior of the 
fourth ventricle is exposed. 
