MAMMALIA 295 



thickened walls laterally, called the thalami ; the thin roof 

 is folded into the velum interpositum, expanded slightly as 

 the corpus habenulare and produced as the pineal body with 

 its stalk. The roof is continued in front into the lamina 

 terminalis, which defines the original front end of the brain. 

 The lamina terminalis passes into the thin floor, which carries 

 the optic chiasma, in front of which is the depression called 

 the optic recess, and behind the tuber cinereum, succeeded 

 by the corpus mammillare and the infundibulum, to which is 

 attached the pituitary body. Anterior and posterior com- 

 missures run across the lamina terminalis and the roof respec- 

 tively, but in addition the side walls meet and fuse, forming the 

 so-called middle commissure. The result is that the cavity 

 of the third ventricle is converted into a ring-shaped vesicle 

 around the region of fusion. 



The mesencephalon is roofed by the optic lobes, resolved 

 into the corpora quadrigemina, and the floor and side walls 

 are formed by the thick peduncles, or crura cerebri, which 

 connect the cerebrum with the rest of the central nervous 

 system. The cavity is the iter, or aqueduct of Sylvius. 



The hindbrain is resolved into the conspicuous dorsal 

 cerebellum, the transverse commissure of the ventral side, 

 termed the pons Varolii, and the posterior medulla oblongata 

 which passes into the spinal cord. 



The cerebellum is a derivative of the extreme forward end 

 of the roof. It grows into a large part of the brain which 

 separates the cerebrum from the hinder part of the skull. 

 Like the cerebrum, it has a cortex of grey matter and a medulla 

 of white matter, and it is folded into lobes separated by fissures, 

 so that in section it has an appearance which has been ex- 

 pressed by the term ' arbor vitae'. The grey matter is divided 

 into an inner layer of multipolar cells, the granular layer, and 

 a superficial layer, the molecular layer, occupied mostly by 

 dendrites. Between them are the conspicuous cells called 

 the cells of Purkinje, which send heavily branching dendrites 

 into the molecular layer. These cells are brought into relation- 

 ship with the rest of the nervous system by the axons which 

 traverse the white substance from three nuclei and enter the 

 peduncles or brachia. There are three peduncles on each 



