FOURTH VENTRICLE 493 



Hum, at the lower end of the roof, is a thin layer of grey 

 matter, called the obex. Above, at the posterior medullary 

 velum, the tela becomes continuous with the pia mater on 

 the lower surface of the vermis of the cerebellum. Laterally 

 the tela is prolonged, on each side, posterior to the restiform 

 body, over the lateral recess, and it forms the stronger part 

 of the wall of that expansion. Between the median part of 

 the tela chorioidea of the fourth ventricle and the pia mater 

 on the lower surface of the vermis of the cerebellum lies the 

 cisterna cerebello-medullaris (O.T. cisterna magna) (Fig. 141). 



Apertures in the Tela Chorioidea of the Fourth Ventricle. 

 In the early stages of development the tela chorioidea and 

 ependyma form an unbroken layer, but at a later period 

 three apertures appear in them. One of the apertures, the 

 apettura medialis ventriculi quarti (O.T. foramen of Magendie\ 

 lies immediately above the obex, at the lower angle of the 

 ventricle, and through it the cavity of the fourth ventricle 

 communicates with the cerebello- medullary portion of the 

 subarachnoid space. The other two apertures lie at the 

 apices of the lateral recesses, immediately posterior to the 

 fila of the glossopharyngeal nerves. 



Chorioid Plexuses of the Fourth Ventricle. The chorioid 

 plexuses are invaginations of the ependyma caused by vascular 

 prolongations of the tela chorioidea. In the lower part of 

 the ventricle they form two parallel bands, one on each side 

 of the median plane, and their lower ends project through 

 the median aperture. At the upper part of the tela chorioidea 

 they communicate together, and then each passes laterally 

 into the corresponding lateral recess and their lateral ex- 

 tremities project through the lateral apertures. 



Dissection. The dissector should now introduce his fingers 

 into the horizontal sulcus of that half of the cerebellum which 

 is still connected with the medulla oblongata and the pons, and 

 tear the upper part of the cerebellum away from the lower part. 

 By that proceeding the manner in which the peduncles enter 

 the white medullary centre, and also, to some extent, the general 

 distribution of their fibres, will be seen. 



Next, separate the remains of the cerebellum from the isthmus, 

 the pons, and the medulla oblongata by cutting through the 

 peduncles at the points where they enter the central white matter. 

 A horizontal section may then be made through the other half 

 of the organ, rather nearer its upper surface than its lower 

 surface. The section will reveal the nucleus dentatus. 



Nucleus Dentatus. The dentate nucleus is a collection 



