NEUROLOGY 





border. The horizontal portion extends transversely across the inferior peduncle, 

 below the striae medullares, and roofs in the lower and posterior part of the lateral 

 recess; it is attached by its lower margin to the inferior peduncle, and partly encloses 

 the choroid plexus, which, however, projects beyond it like a cluster of grapes; ard 

 hence this part of the tsenia has been termed the cornucopia (Bochdalek) . The obex 

 is a thin, triangular, gray lamina, which roofs in the lower angle of the ventricle and 

 is attached by its lateral margins to the clavae. The tela chorioidea of the fourth 

 ventricle is the name applied to the triangular fold of pia mater which is carried 

 upward between the cerebellum and the medulla oblongata. It consists of two 

 layers, which are continuous with each other in front, and are more or less adherent 

 throughout. The posterior layer covers the antero-inferior surface of the cere- 

 bellum, while the anterior is applied to the structures which form the lower part 

 of the roof of the ventricle, and is continuous inferiorly with the pia mater on the 

 inferior peduncles and closed part of the medulla. 



Corpora 

 quadrigemina 

 Cerebral 

 peduncle 

 Anterior 

 medullary 

 velum 

 Ependymal 

 lining of 

 ventricle 



Posterior 

 medullary velum 



Choroid plexus 



Cisterna cerebellomedullaris of 

 subarachnoid cavity 



Central canal mm ,^ 



Cisterna pontis of 

 subarachnoid cavity 



Fio. 708. Scheme of roof of fourth ventricle. The arrow is i n the foramen of Majendie. 



Choroid Plexuses. These consist of two highly vascular inflexions of the tela 

 chorioidea, which invaginate the lower part of the roof of the ventricle and are 

 everywhere covered by the epithelial lining of the cavity. Each consists of a ver- 

 tical and a horizontal portion : the former lies close to the middle line, and the latter 

 passes into the lateral recess and projects beyond its apex. The vertical parts of 

 the plexuses are distinct from each other, but the horizontal portions are joined 

 in the middle line; and hence the entire structure presents the form of the letter T, 

 the vertical limb of which, however, is double. 



Openings in the Roof. In the roof of the fourth ventricle there are three openings, 

 a medial and two lateral: the medial aperture (foramen Majendii), is situated imme- 

 diately above the, inferior angle of the ventricle; the lateral apertures, (foramina 

 of Luschka are found at the extremities of the lateral recesses. By means of these 

 three openings the ventricle communicates with the subarachnoid cavity, and the 

 cerebrospinal fluid can circulate from the one to the other. 



Rhomboid Fossa (fossa rhomboidea; "floor" of the fourth ventricle) (Fig. 709). 

 The anterior part of the fourth ventricle is named, from its shape, the rhomboid 

 fossa, and its anterior wall, formed by the back of. the pons and medulla oblongata, 

 constitutes the floor of the fourth ventricle. It is covered by a thin layer of gray 



