698 



THE NERVOUS SYSTEM. 



foramen of Majendie, and of a transverse part, which passes outward into the lateral 

 recess of the ventricle as far as the foramina of Key and lletzius. The two 

 plexuses present the form of a T, the vertical limb of which is, however, double, 

 ~]| . The tela does not form a complete membrane, for in it there are three 

 openings, one in the middle line at the inferior angle of the ventricle, just 

 above the position of the opening of the central canal of the cord ; this is the fora- 

 men of Majendie : the other two are at the extremities of the lateral recesses of the 

 ventricle, and are named the foramina of Key and Retzius (see page 642). 

 Through these foramina the ventricles of the brain communicate with the 

 subarachnoid space. 



The obex is a thin triangular lamina of gray matter, continuous below with the 

 anterior gray commissure of the cord, which fills in the angle between the two 

 diverging funiculi graciles for a short distance. 



The ligula (tcenice) are narrow bands of white matter, which project from the 

 internal border of the funiculi graciles. They at first run upward and forward, 



Corpora qiiadriyemina. 



Processus e cerelello ad testes. 

 Valve of Vieussens. 



Eminentia teres. 



Conductor sonorus. 

 Strise acusticse. 



Locus cteruleus. 

 Fovea superior. 



Trigonum hypoglossi. 



Ala cinerea 



Trigonum acustid. 

 Clava. 



Tuberculum cunealum. 

 Lateral column. 



FIG. 371. Floor of the fourth ventricle. Diagrammatic. 



ana then turn outward over the restiform bodies, as far as the latera recesses of the 

 ventricle. Their inner borders are continuous with the epithelial roof of the 

 ventricle. 



The floor of the fourth ventricle (Fig. 371) is rhomboidal in shape, and is 

 traversed by a vertical median fissure, the sulcus longitudinalis medianus. At its 

 widest part, opposite the level of the lateral recesses, it is marked by some trans- 

 verse white lines, the strice medullaris or strice acusticce. These consist of white 

 fibres, which emerge from the longitudinal sulcus, and pass outward across the floor 

 of the ventricle. 



These striae divide the floor of the ventricle into two triangles, inferior and 

 superior. The inferior triangle, or lower half of the floor, presents above an 

 angular groove, ihefovea inferior, the apex of which is at the striae, while the two 

 limbs diverge below, and form the sides of a triangular, dark area, termed the ala 

 cinerea, which becomes elevated into a prominence below (eminentia cinerea). 

 This area corresponds with the nuclei of the vagus and glosso-phai'yn- 

 geal nerves, and is therefore termed the trigonum vagi. A second triangular area 

 lies between the inner limb of the fovea and the median sulcus ; its base is directed 



