NUCLEUS DENTATUS 539 



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 they 

 are perforated by three apertures. One of the apertures, the 

 apertura 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 vascular invaginations of the ependyma beneath 

 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 medial aper- 

 ture. At the upper part of the tela chorioidea they com- 

 municate together, and then each passes laterally into the 

 corresponding lateral recess and their lateral extremities 

 project through the lateral apertures. 



Dissection. The dissector should now introduce his fingers into the 

 great 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 this 

 side of the organ away from the lower part. By this 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. 



When these have been demonstrated this half of the cerebellum should also 

 be separated 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. This will reveal the nucleus dentatus. 



Nucleus Dentatus of the Cerebellum. This is a collection 

 of grey matter, embedded in the white medullary centre of 

 the lateral hemisphere of the cerebellum. It presents an 

 appearance very similar to a nucleus which lies in the olive of 

 the medulla oblongata. It is a thin lamina of grey matter, 

 which appears on section as a wavy line folded upon itself, 

 so as to form a crumpled grey capsule with a mouth open 

 towards the median plane. The greater number of the fibres 

 which build up the brachium conjunctivum issue from its 

 mouth. 



There are other smaller isolated nuclei of grey matter in the white 

 medullary centre of the cerebellum, but these cannot, as a rule, be 



