FOURTH VENTRICLE 537 



rhomboidal fossa lies in the medulla oblongata; the inter- 

 mediate "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 loiver part of the rhomboidal 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. These torn margins 

 are the tcenia of the fourth ventricle. The intermediate part of 

 the rhomboidal 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 

 rhomboidal fossa lies posterior to the pons and between the 

 brachia conjunctiva. At its upper end it becomes continuous 

 with the aquseductus cerebri of the mid-brain. 



The floor , or anterior boundary, of the fossa rhomboidalis 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 eminentia medialis. In the upper part of the 

 fossa the eminentia medialis occupies practically the whole 

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

 intermediate portion of the fossa a nodular eminence, the 

 colliculus facialis, appears on its surface; below the colliculus 

 it narrows rapidly, and it terminates, below, in a pointed tri- 

 angular process called the trigonum hypoglossi. The medial 

 eminence is bounded laterally by a sulcus, the sulcus liinitans. 

 In the upper region, along the lateral border of the sulcus 

 limitans, is a narrow bluish tinted area called the locus 

 cccruleus ; the colour of this area is due to a subjacent col- 

 lection of pigmented cells which constitute the substantia 



