218 DISSECTION OF THE BRAIN. 



(fovea posterior) ; the lower is the nucleus of the vagus and glosso-pharyn- 

 geal nerves, and the upper is the nucleus of the auditory nerve. Running 

 into the lower part of the vagus nucleus, is the nucleus of the accessory 

 portion of the spinal accessory nerve. (See p. 189.) 



In the upper half of the space some other nerves take origin from nu- 

 clei, but there is only one projection. This is placed over the common 

 nucleus of the sixth and the facial nerve : it is a rounded elevation on the 

 outer part of the eminentia teres, about a line above the white cross stria) 

 on the floor, and close behind the fovea anterior. 



The fourth ventricle communicates at the upper part with the third 

 ventricle through the Sylvian aqueduct ; and with the subarachnoid space 

 of the cord and brain, through an aperture in the pia mater intervening 

 between the medulla and the cerebellum : laterally, the ventricular space 

 is extended for a short distance between the cerebellum and the side of 

 the medulla oblongata. 



The lining of the other ventricles is prolonged into this by the aperture 

 of communication with the third. Covering the floor is a columnar epi- 

 thelium, which is continuous with that in the upper part of the central 

 canal of the spinal cord (Clarke). 



In this ventricle is a vascular fold chorotd plexus, on each side, simi- 

 lar to the body of the same name in tiie other ventricles. It is attached 

 to the inner surface of the membrane (pia mater) which closes the ventricle 

 between the medulla and the cerebellum, and it extends upwards on the 

 side of the opening into the sub-arachnoid space. Its vessels are supplied 

 by the inferior cerebellar artery. 



Gray matter of fourth ventricle. The gray matter forms a surface- 

 covering for the floor of the fourth ventricle. It is continuous below with 

 the gray commissure of the cord, and extends upwards to the aqueduct of 

 Sylvius (p. 187). The special nuclei have been referred to already 

 (p. 217). 



