35 



NERI/OUS SYSTEM. 



toward the floor of the ventricle, forming the choroid plexus 

 of the fourth ventricle. In the middle the roof of the fourth 

 ventricle is the cerebellum, while craniad the roof is the velum 

 medullare anterius (Fig. 143, /). This is a thin layer just 

 craniad of the cerebellum, connecting it with the corpora quad- 

 rigemina, and attached laterally to the brachia conjunctiva. 

 Here the fourth ventricle narrows craniad and becomes con- 

 tinuous with the slender aqueductus cerebri (Fig. 143, /) 

 (aqueduct of Sylvius). The narrowed portion of the brain is 

 frequently known as the isthmus rhombencephali. 



FIG. 141. DORSAL SURFACE OF MYELENCEPHALON, MESENCEPHALON, AND 



DlENCEPHALON. 



The cerebellum and the greater part of the cerebral hemispheres have been re- 

 moved, a, posterior sulcus of cord; , posterior lateral sulcus; c, fasciculus gracilis; 

 d, clava; e, fasciculus cuneatus; f, area ovalis; g. fasciculus cuneatus lateralis; ^, 

 fossa rhomboidea or floor of fourth ventricle; /, projection formed by origin of audi- 

 tory nerve;/, facial nerve; , cut end of brachium conjunctivum; /, cut end of bra- 

 chium pontis (/'. brachium pontist; m, velum medullare anterius; , fourth nerve; 

 o, depression marking caudal end of aqueductus cerebri (aqueduct of Sylvius); /, 

 caudal corpora quadrigemina (colliculi posteriores) ; q, cranial corpora quadrigemina 

 (colliculi anteriores); r, brachium quadrigeminum inferius; s, corpus geniculatum 

 mediale; t, thalamus; u, striae medullares; v, third ventricle; w, pulvinar; x, corpus 

 geniculatum laterale; y, corpus striatum; z, outlines of olfactory bulbs. I, boundary 

 between hemispheres and 'tween-brain; 2, pineal body. 



The fourth cranial nerve (Fig. 141, n; Fig. 138, IV) 

 (N. trochlearis) arises from the brain at the craniolateral angle 

 of the velum medullare anterius. 



