

THE HIND-BRAIN OR RHOMBENCEPHALON 



799 



substance continuous with that of the medulla spinalis; superficial to this is a thin 

 lamina of neuroglia which constitutes the ependyma of the ventricle and supports 

 a layer of ciliated epithelium. The fossa consists of three parts, superior, inter- 

 mediate, and inferior. The superior part is triangular in shape and limited laterally 

 by the superior cerebellar peduncle; its apex, directed upward, is continuous with 

 the cerebral aqueduct; its base it represented by an imaginary line at the level of the 

 upper ends of the superior fovese. The intermediate part extends from this level 

 to that of the horizontal portions of the taenise of the ventricle; it is narrow above 

 where it is limited laterally by the middle peduncle, but widens below and is pro- 

 longed into the lateral recesses of the ventricle. The inferior part is triangular, 

 and its downwardly directed apex, named the calamus scriptorius, is continuous 

 with the central canal of the closed part of the medulla oblongata. 



Frenulum veli 



Tcenia pontis 



Trochlear nerve 

 Ant. medullary velum 

 Superior peduncle 



Nucleus dentatus 



Superior fovea 

 Collicuhis facialis 



Striae medullares <^ 



Area acustica, 

 Trigonum hypoglossi 

 Ala cinerea 

 Tcenia of fourth ventricle 





' Funiculus separans 

 \Area postrema 



i Clava 

 FIG. 709. Rhomboid fossa. 



The rhomboid fossa is divided into symmetrical halves by a median sulcus 

 which reaches from the upper to the lower angles of the fossa and is deeper below 

 than above. On either side of this sulcus is an elevation, the medial eminence, 

 bounded laterally by a sulcus, the sulcus limitans. In the superior part of the fossa 

 the medial eminence has a width equal to that of the corresponding half of the 

 fossa, but opposite the superior fovea it forms an elongated swelling, the colliculus 

 facialis, which overlies the nucleus of the abducent nerve, and is, in part at least, 

 produced by the ascending portion of the root of the facial nerve. In the inferior 

 part of the fossa the medial eminence assumes the form of a triangular area, the 

 trigonum hypoglossi. When examined under water with a lens this trigone is seen 

 to consist of a medial and a lateral area separated by a series of oblique furrows; 

 the medial area corresponds with the upper part of the nucleus of the hypoglossal 

 nerve, the lateral with a small nucleus, the nucleus intercalates. 



The sulcus limitans forms the lateral boundary of the medial eminence. In 

 the superior part of the rhomboid fossa it corresponds with the lateral limit of the 



