6oo 



NERVOUS SYSTEM 



bulb and the cerebellum. It is lozenge-shaped, the acute angles being 

 above and below. The upper angle extends to the upper border of the 

 pons, and the lower angle, to the lower border of the olivary bodies. 

 The triangles which form this lozenge are of nearly equal size. The 

 superior triangle is bounded laterally by the superior peduncles of the 

 cerebellum, as they converge to meet at the corpora quadrigemina. 

 The inferior triangle is bounded laterally by the funiculi graciles and 

 the restiform bodies of the medulla, which diverge at its lower angle. 

 The arched roof of the ventricle is formed by the valve of Vieussens, 

 which is stretched between the superior peduncles of the cerebellum 

 and covers the anterior triangle, and the cerebellum, which covers the 



posterior triangle. Beneath 

 the cerebellum is a reflection 

 of the pia mater. The fourth 

 ventricle communicates above 

 with the third ventricle by the 

 aqueduct of Sylvius ; below 

 with the subarachnoid space, 

 by the foramen of Magendie ; 

 and by a small opening below, 

 with the central canal of the 

 cord. The floor of the ventri- 

 cle is formed by the posterior 

 surface of the pons above and 

 the bulb below. It presents 

 a fissure in the median line, 

 which terminates below in the 



5, s, superior peduncles (divided) of the cerebellum; 6, CalamUS SCriptOHUS. By the 



6, bands to the side of the crura cerebri ; 7,7, lateral sides of the median fissure 

 grooves of the crura cerebri ; 8, corpora quadrigemina. 



are the fasciculi teretes, which 



correspond to the intermediary fasciculi of the bulb. Little eminences 

 in the floor indicate the situation of nuclei of origin of cranial nerves. 

 The floor is composed mainly of a layer of gray matter, continuous 

 with the gray commissure of the cord. The lower portion of the floor 

 is marked by transverse lines of white matter emerging from the median 

 fissure. 



The two lateral halves of the posterior portion of the bulb are con- 

 nected together by fibres arising from the gray matter of the lateral 

 tracts, or intermediary fasciculi, passing obliquely, in a curved direction 

 from behind forward, to the raphe in the median line. There also are 

 fibres passing from before backward to form a posterior commissure, 

 and fibres arising from the cells of the olivary bodies, which connect 



Fig. 152. Floor of the fourth ventricle (Hirschfeld). 



i, median fissure, between the fasciculi teretes; 

 2, transverse white striae; 3, inferior peduncle of the 

 cerebellum; 4, posterior pyramid (funiculus gracilis) ; 



