FOURTH VENTRICLE 491 



separate one half of the cerebellum, which must be laid aside 

 for the present, but must be preserved for future use. 



When the dissection is completed the dissectors will be 

 able to recognise that the anterior part of the cavity of the 

 fourth ventricle is rhomboidal in form. It constitutes the 

 so-called rhomboid fossa, which is surrounded by the lateral 

 boundaries of the ventricle and closed anteriorly by the pons 

 and the dorsal surface of the ventral part of the upper 

 half of the medulla oblongata. Only the lower part of 

 the rhomboid fossa lies in the medulla oblongata ; the 

 middle part is in the metencephalon, that is, it lies anterior 

 to the cerebellum and posterior to the lower part of the pons ; 

 and the upper part is in the isthmus rhombencephali. 



The lower part of the rhomboid fossa is triangular in out- 

 line, and its inferior angle is continuous with the central canal 

 of the lower part of the medulla oblongata. The anterior 

 boundary or floor of this part of the fossa is marked by a 

 number of converging sulci, and is called the calamus 

 scriptorius. Along the lateral margins of the lower part of 

 the fossa will be seen the remains of the torn epithelial roof 

 of the lower part of the fourth ventricle. The torn margins 

 are the tcenice of the fourth ventricle. The middle part 

 of the rhomboid fossa is separable into a lower wider part, 

 which is prolonged laterally, on each side, below and posterior 

 to the restiform body, as the lateral recess of the fourth 

 ventricle. The upper section of the intermediate part of 

 the fossa is bounded laterally by the brachia pontis and is 

 much narrower than the lower part. The upper part of the 

 rhomboid fossa lies posterior to the pons and between the 

 brachia conjunctiva. At its upper end it becomes continuous 

 with the aquaeductus cerebri of the mid-brain. 



The floor, or anterior boundary, of the fossa rhomboidea is 

 the floor, or anterior boundary, of the fourth ventricle. In 

 the upper part of its extent it is formed by the posterior 

 surface of the pons, and in the lower part by the posterior 

 surface of the ventral part of the upper portion of the medulla 

 oblongata. It is divided into lateral portions by a median 

 sulcus which is deeper below, in the region of the calamus 

 scriptorius, and shallower above. On each side of the median 

 sulcus is the emhientia medialis. In the upper part of the 

 fossa the eminentia medialis occupies practically the whole 

 of each half of the floor ; in the upper part of the middle 



