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, »*) excavated in the dorsal region of the 

 medulla oblongata, and called the /ourt/i 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 'tween brain (Di). 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 



