358 



ANATOMY OF THE RABBIT 



{d) The posterior medullary velum (velum medullare pos- 

 terius) underlies the posterior margin of the cerebellum, 

 and extends backward over the triangular space enclosed 

 by the walls of the fourth ventricle. It is a more delicate 

 membrane than the anterior medullary velum and supports 

 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, 

 (e) On the ventral surface (Fig. 121), the pons forms a broad 

 band extending transversely across the brain and upward 

 into the supports (peduncles) of the cerebellum, its fibres 

 being distributed to the cortex of the cerebellar hemispheres. 

 Its surface is divided into two parts by a median depres- 

 sion, the sulcus basilaris, occupied by the basilar artery. 

 It is not really a commissure but is part of the pathway 

 connecting the cerebral hemispheres with those of the 

 cerebellum. 



The portion of the brain 

 caudal to the posterior margin 

 of the pons is the medulla 

 oblongata. 

 (/) The anterior median fissure 

 of the spinal cord ends at the 

 posterior margin of the pons 

 in a faint depression, the 

 foramen caecum. 

 (g) The anterior funiculus of the 

 spinal cord is continuous with 

 a narrow band on the ventral 

 surface of the hindbrain, the 

 pyramid, which may be ob- 

 served extending backwards 

 on each side of the middle line 

 from the posterior margin of 

 the pons. The pyramids are 

 really the reduced continu- 

 ation backwards of the fibre- 



zvvvy 



Fig. 121. The rhombencephalon. 

 Ventral view (the cerebellum not 

 figured). 



ct, trapezoid body; fc, foramen 

 caecum; tic, cervical flexure; fma, 

 anterior median fissure; p, pons; 

 PC cerebral peduncle (mesence- 

 phalon); py, pyramid. 



Ill, oculomotor nerve; IV, troch- 

 lear; V^ portio major of the trige- 

 minus; V^, portio minor; VI, ab- 

 ducens; VII, facial; VIII, acoustic; 

 IX-XI, glossopharyngeal, vagus, 

 and spinal accessory group; XII, 

 hypoglossal; ci, first cervical spinal. 



