198 ANIMAti PHYSIOLOGY, 



the crura cerebri, and proceeding to the corpora quadrigemiim 

 from the cerebellum; these are the superior crura cerebelli 

 already alluded to ; and, between these, a thin lamina, called 

 valve of Vieussens, limited by the cerebellum behind and the 

 corpora quadrigemina in front, forms the roof of the fore part 

 of the fourth ventricle, as that hollow is continued forwards 

 into a narrow canal or iter which passes beneath the corpora 

 quadrigemina, and opens in front of them. 



By reflecting the hemispheres well forwards off the crura, 

 a pair of large elevations, the optic thalami, will be exposed 

 in front of the corpora quadrigemina ; and, by dividing the 

 corpus callosum and other structures, so as to permit more 

 complete reflection of the hemispheres, still another pair of 

 elevations, the corpora striata, will be seen in front of the 

 optic thalami, and external to them. A soft body, about the 

 size of a pea, the pineal body, will be likewise noticed 

 attached by a slender connection in front of the corpora 

 quadrigemina, and overhanging them, imbedded in pia mater. 

 It may be mentioned of this structure that it is remarkable 

 in being present in all the divisions of the vertebrata, 

 although in the lower forms represented by little else than 

 vascular tissue, and in man consisting of degenerated brain 

 structure. 



147. On the under surface, or base of the brain, the crura 

 cerebri are crossed by two bands of fibres, which can be 

 traced round the crura to the back parts of the optic thalami, 

 and to the corpora quadrigemina. These bands, the optic 

 tracts, meet in the middle line, and enclose a lozenge -shaped 

 interval between the crura ; at their place of junction, the 

 optic commissure, they exchange fibres, and in front of this 

 they diverge as the optic nerves, or nerves of sight, to the 

 eyeballs. In the lozenge-shaped interval, between the crura, 

 are seen a pair of white bodies like small peas, the corpora 

 albicantia, which are closely connected with the -optic thalami, 

 and in front of them a funnel of membranous grey brain 

 matter, the infundibulum, leading to a firm body which lies 

 in the sella turcica of the sphenoid bone, and is called the 

 pituitary body a structure certainly not nervous, of function 

 quite unknown, but well developed in all divisions of the 

 vertebrata. Outside the fore part of the lozenge-shaped 



