126 



THE NERVOUS SYSTEM 



tion with the subarachnoid space, and cerebrospinal fluid may escape from the 

 former into the latter. 



The floor of the fourth ventricle is known as the rhomboid fossa and is formed 

 by the dorsal surfaces of the pons and open part of the medulla oblongata, which 

 are continuous with each other without any line of demarcation and are irreg- 

 ularly concave from side to side (Figs. 89, 91). The fossa is widest opposite the 

 points where the restiform bodies turn dorsally into the cerebellum; and it 

 gradually narrows toward its rostral and caudal angles. The lateral boundaries 



Pineal body 



~ Superior colliculus 



Inferior colliculus 

 Cerebral peduncle 

 Trochlear nerve 

 Median sulcus 

 Locus caruleus 

 Facial colliculus 

 Medial eminence 

 Sulcus limitans 



Lateral recess 



Stri(B medtillares 



Tcenia 



Trigonum hypoglossi 



Cuneate tubercle 



Tuber culum cinereum 



Clava 



Posterior median fissure 



Posterior intermediate 



sulcus 

 'Posterior lateral sulcus 



Medial geniculate body--*- 



Inferior quadrigeminal T~ 



brachium 



Frenulum veli 



A nterior medullary velum 



Brachium conjunctivum 



Brachium pontis*--^ 

 Restiform body- 



t 



Superior fovea 



Area acustica-<-=:~ 



Inferior fovea 



Restiform body 



Ala cinerea """ 



Funiculus separans"'" 



Area postrema-"'" 



Obex-''"' 



Funiculus gracilis -~~~ 

 Funiculus cuneatus~ " 



Fig. 89. Dorsal view of human brain stem. 



of the fossa, which are raised some distance above the level of the floor, are 

 formed by the following structures: the brachia conjunctiva, restiform bodies, 

 cuneate tubercles, and clava. Of the four angles to the rhomboid fossa, two 

 are laterally placed and correspond to the lateral recesses. At its caudal angle 

 the ventricle is continuous with the central canal of the closed part of the me- 

 dulla oblongata, and at its rostral angle with the cerebral aqueduct. Joining 

 the two last named angles there is a median sulcus which divides the fossa into 

 two symmetric lateral halves. 



The rhomboid fossa is arbitrarily divided into three parts. The superior 



