250 A MANUAL OF ANATOMY. 



The Fourth Ventricle and the Posterior and Lateral 

 Tracts of the Medulla. Figs, 39, 54. 



The fourth ventricle is an elongated, diamond-shaped 

 space communicating anteriorly by means of the aqueduct 

 of Sylvius with the third ventricle and posteriorly with the 

 minute central canal of the spinal cord. The lateral angles 

 of the space are in the recesses left by the junction of the 

 peduncles at the side of the cerebellum. 



The roof of the ventricle is formed from before back- 

 ward by the valve of Vieussens, the under surface of the 

 cerebellum, and the portion of pia which forms the choroid 

 plexus of the fourth ventricle and extends from the postero- 

 inferior surface of the cerebellum to the upper surface of 

 the lower part of the medulla. Remember that this por- 

 tion of pia is composed of loops of capillaries, over which 

 is laid the layer of cells which represents the attenuated 

 roof of the posterior afterbrain, and consequently the 

 plexus is shut out from the ventricular cavity. This layer 

 of pia with its choroid plexus and lining membrane of epi- 

 thelium is called the tela choroidea inferior. Also, re- 

 member that the tela inferior is perforated in the middle 

 line and near the medulla by the foramen of Magendie, and 

 in the lateral recesses by narrow slits of the foramina of 

 Key and Retzius. Through these apertures a free com- 

 munication is offered to the cerebrospinal fluid to pass from 

 the subarachnoidean spaces into the cavity of the ventricles 

 of the brain. 



The Lateral Boundaries of the Fourth Ventricle. 

 These are : In front, the superior peduncles ; in the middle, 

 the junction of the superior, middle, and inferior peduncles 

 with each other and with the cerebellum ; the posterior 

 portion, by the inferior peduncles (restiform tracts), the 

 fasciculi graciles, and their clavae. 



