BRAIN 157 



greater diameter than the cord, and is made up of 

 several very distinct parts or divisions. The hinder- 

 most 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 tri- 

 angular body (D, ch. plx 2 ), of a reddish colour in the 

 fresh condition, called the posterior 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, i> 4 ) excavated in the dorsal region of 

 the medulla oblongata, and called the fourth ventricle. 

 The apex of the cavity, which is directed backwards, 

 opens into the central canal of the spinal cord (Fig. 50, 

 t; 4 , c. c), and 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 (Opt. v), communi- 

 cating with a narrow median passage, the iter (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 struc- 

 ture, the diencephalon or 'tween-brain (Di). On its 

 upper surface is a small rounded vascular body, the 

 anterior choroid plexus (Fig. 49, D, ch. plx 1 ), formed, 



