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 

 CoUiculi. 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 of 

 the nerve is usually to be found 

 on the lateral surface of the 

 cerebral peduncle. 



(d) The posterior medullary 

 velum (velum medullare pos- 

 terius) underlies the,, posterior 

 margin of the cerebellum, and 

 extends backward over the tri- 

 angular space enclosed by the 

 walls of the fourth ventricle. 

 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 brain, in which case the interior of the 

 fourth ventricle is exposed. , 



Fig. 8s. The rhombencephalon'. 

 Ventral view (the cerebellum not 

 figured). 



c.t., trapezoid body; f.c, foramen 

 caecum; fl.c, cervical flexure; f.'m.a., 

 anterior median fissure; p.. pons; 

 P.C., cerebral peduncle (mesence- 

 phalon); py., pyramid. 



Ill, oculomotor nerve; IV, troch- 

 lear; .VI, poriio major of the trige- 

 minus; V2, portio minor; VI, ab- 

 'ducens; VII, facial; Vlli, acoustic. 

 IX-XI, glossopharyngeal, vagus, and 

 spinal accessory ^roup; , XII, hypo- 

 CIi first cervical spinal. 



