THE FOURTH VENTRICLE. 313 



Lesions in the medulla are very fatal and death usually occurs 

 before any sensory or motor phenomena can be observed; but 

 rarely the pyramidal tracts alone have been involved or the pyram- 

 idal tracts together with one or more of the roots of the ninth to 

 the twelfth cerebral nerves. In the last case, crossed paralysis 

 is produced, as in the pons, affecting the cerebral nerves on the 

 same side and the opposite spinal nerves. In progressive bulbar 

 paralysis the motor nuclei of the medulla are involved as a pre- 

 liminary to the degeneration of the anterior gray columna in the 

 spinal cord. 



RHOMBENCEPHALON. 

 SECTION IV. THE FOURTH VENTRICLE. 



The common cavity of the rhombencephalon is the* fourth 

 ventricle (venlriculus quartus) (Fig. 118). The fourth ventricle 

 is contained chiefly in the pons and medulla, and is ventral to the 

 cerebellum (Fig. 79). It is broadest at the junction of the pons 

 and medulla (Figs. 86 and 91). Above and below that junction, 

 it gradually contracts to the size of the cerebral aqueduct and 

 central canal of the spinal cord, with which it is continuous. 

 Inferiorly it communicates through its roof with the subarachnoid 

 space via three apertures, a median and two lateral. The 

 fourth ventricle is a gable-roofed chamber with a diamond- shaped 

 floor. The gables (Fig. 79) are directed lateralward and are 

 prolonged in tunnel-like extensions around the restiform body. 

 The long axis of the ventricular floor (Figs. 86 and 96) is parallel 

 with the spinal cord, and extends from the superior extremity of 

 the pons to the middle of the medulla. The transverse axis 

 coincides with the junction of the pons and medulla. Thus the 

 superior triangle of the floor is formed by the pons; the inferior, 

 by the medulla oblongata. The fourth ventricle is lined with 

 ependyma, which is complete throughout, except in the roof of 

 the inferior part, where below the inferior medullary velum only 

 the epithelial layer is present. 



Boundaries. The floor is formed by the pons and medulla. 



