2QO THE RHOMBENCEPHALON. 



it, termed the chorioid tela of the fourth ventricle, this roof epithe- 

 lium is perforated in the median line near the obex by a foramen, 

 the median aperture (apertura mediana ventriculi quarti, Magendi) 

 and over each lateral recess by the lateral aperture (apertura 

 later alls ventriculi quarti, Key and Retzii). These foramina estab- 

 lish communication between the subarachnoid space and the ven- 

 tricle. On either side of the median line there is a longitudinal 

 invagination of the epithelial lamina into the ventricle and a 

 similar transverse one just below the inferior medullary velum, 

 both of which are occupied by a vascular fold of pia mater. This 

 fold constitutes the chorioid plexus of the fourth ventricle (plexus 

 chorioideus ventriculi quarti). If the roof epithelium be torn 

 away, as it usually is with the pia, a rough line of separation is 

 seen winding over the restiform body. That line is the tctnia 

 of the fourth ventricle. Two layers of ependyma form it. 



When the roof epithelium is thus removed, the lower triangle 

 of the floor of the fourth ventricle is brought into view (Figs. 

 86 and 96). Notice the median longitudinal furrow bounded by 

 the eminenticB mediates, which form the calamus scriptorius; then 

 the little fossa in the sulcus limitans, called the fovea inferior, 

 situated lateral to the middle of the pen, and the ala cinerea (tri- 

 gonum vagi), whose superior angle is formed by the fovea inferior; 

 and, last, the large lateral area, located above the ala cinerea 

 external to the eminentia medialis, and crossed by the nearly 

 transverse medullary strice. This region is called the area acus- 

 tica. 



WHITE MATTER OF MEDULLA. 



The medulla is made up of white and gray matter, which to- 

 gether bound ventrally and laterally the inferior part of the fourth 

 ventricle, and surround the upper extremity of the sixth ventricle 

 (Figs. 92 and 95). 



For the most part the white matter of the medulla is continu-. 

 ous with the longitudinal fibers of the pons and restiform bodies, 

 above, and with the spinal cord, below; the bulbar roots of the 

 eighth to the twelfth cerebral nerves, and many decussating or 



