X BRAIN 157 



tinct parts or divisions. The hindermost division is called 

 the bulb, or medulla oblongata {Med. obi) ; this appears to be 

 simply a widening of the spinal cord {Sp. cd), except that 

 on its dorsal surface is a triangular body (D, ch. plx^) of a 

 reddish colour in the fresh condition, and called the pos- 

 terior choroid plexus: it is simply a thickening of the pia 

 mater containing abundant blood-vessels. 



The choroid plexus forms a kind of lid to a triangular 

 cavity (A and D, w*) excavated in the dorsal region of the 

 medulla oblongata, and called \k\s. fourth ventricle. The apex 

 of the cavity, which is directed backwards, opens into the 

 central canal of the spinal cord (Fig. 50, »*, c.c), and the 

 the fourth ventricle is to be looked upon simply as the 

 anterior part of the central canal which has become 

 widened out and is covered only by a thickened portion 

 of the pia mater. 



The fourth ventricle is bounded in front by a narrow 

 ledge of nervous matter (Fig. 49, Cb), which would be hardly 

 worthy of being considered as a special division of the 

 brain but for the fact that the corresponding part in many 

 animals — e.g., dogfish, rabbit, man — is a large and important 

 structure. It is called the cerebellum. 



In front of the cerebellum comes a pair of rounded 

 elevations, the optic lobes (Figs. 49 and 50, Opt. I). Each 

 contains a cavity, the optic ventricle (Fig. 50, Opt. v), 

 communicating with a narrow median passage, the iter 

 (Figs. 49 D and 50, i), which is continuous behind with the 

 fourth ventricle. The bulb is continued forwards beneath 

 the optic lobes as the crura cerebri (Cr. C). 



In front of the optic lobes is an unpaired structure, the 

 diencephalon or Hween brain {Dt). On its upper surface is 

 a small rounded vascular body, the anterior choroid plexus 

 (Fig. 49 A, D, ch. plx ^), formed, like the posterior choroid 



