THE BABBIT. 103 



should be carefully borne in mind by the student. The 

 openings from the two lateral ventricles into the third are 

 known as the foramina of Monro (fig. 56). 



The roof of the first vesicle is prolonged up into a conical 

 top, at the end of which a mysterious body, the pineal body, 

 is found ; its floor is similarly prolonged down into a funnel 

 or infundibulum, with the equally mysterious pituitary 

 body ; its side- walls become the optic thalami. The roof 

 of the second vesicle forms the optic lobes, or corpora 

 quadrigemina ; its floor (consisting largely of nerve-fibres 

 running towards the cerebral hemispheres) has its right and 

 left halves, each known as a cms (plural crura) cerebri. The 

 roof of the hind-brain has its anterior portion greatly 

 developed into the cerebellum (consisting of five lobes a 

 median vermis, lateral lobes, and still more lateral jlocculi), 

 while its posterior portion, on the contrary, is extremely 

 thin. The part of its floor beneath the cerebellum is called 

 the pons Yarolii, or more briefly, the pons ; while the 

 hinder part is the medulla oblong ata, more conveniently 

 called the bulb: this gradually passes back into the 

 spinal cord. The cavity of the bulb (fourth ventricle) is 

 very large, and has no roof of nerve-tissue (only connec- 

 tive tissue). Traced back, this cavity narrows and becomes 

 divided into the dorsal fissure and central canal of the cord. 



7. White and Grey Matter in the Brain. We have seen ( 3) 

 that the nerve-fibres to and from the brain occupy definite positions 

 in the white matter of the spinal cord. It is a very remarkable fact 

 that somewhere in the course of its journey every such fibre crosses 

 from one side to the other (left to right or right to left). This 

 crossing is called decussation: it happens to certain fibres in the 

 spinal cord itself, but the main region of its occurrence is in the 

 bulb. 



If we took a series of transverse sections from the spinal cord 

 forwards through the bulb, we should find the first important change 

 in the appearance of the sections to be due to this decussation. 

 Besides this we find that although grey matter continues to form a 

 central mass around the central canal all the way to the front of 

 the mid-brain, scattered patches of grey matter also appear among 

 the white matter, and the whole appearance of the transverse 

 section loses its simplicity. The central canal becomes more dorsal 

 in position, its roof thins away, and, combining with the dorsal 

 fissure, it enlarges into the fourth ventricle. 



Finally in the cerebral hemisohercs. and in the cerebellum, the 



