THE MEDULLA OBLONGATA. 



629 



The fourth ventricle is the cavity between the pons Varolii, the medulla 

 oblongata and 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 ven- 

 tricle 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 com- 

 municates 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 medulla 

 below. It presents a fissure in the median line, which terminates below 

 in the calamus scriptorius. By the sides of the median fissure, are the 

 fasciculi teretes, which correspond to the intermediary fasciculi of the me- 

 dulla. 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 

 medulla are connected 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 



are also fibres passing from be- FIG. 232. Floor of the fourth ventricle (KirschMd). 



fnrp hflpkwnvl tn fnrm n nn 1, median fissure, between the fasciculi teretes ; 2, trans- 

 DdCKWdlCl, t( verge ^ whjte ^ tri8e . 3 ^^0,. peduncle of the cerebel- 



terior commissure, and fibres 

 arising from the cells of the 

 olivary bodies, which connect 

 the gray substance of the lateral halves. Commissural fibres also connect the 

 gray matter of the lateral tracts with the corpora dentata of the olivary bod- 

 ies, and the olivary bodies with the cerebellum, their fibres forming part of 

 the inferior peduncles of the cerebellum. In addition it is probable that 



lum ; 4, posterior pyramid (funiculus gracilis) ; 5, 5, su- 

 perior peduncles (divided) of the cerebellum ; 6, 6, bands 

 to the side of the crura cerebri ; 7, 7, lateral grooves of 

 the crura cerebri ; 8, corpora quadrigemina. 



