550 



THE NEKVOUS SYSTEM. 



Fossa Rhomboidea (floor of the fourth ventricle). In its inferior part the floor 

 of the fourth ventricle is formed by the dorsal surface of the ventral part of the 

 medulla oblongata, whilst in its superior part it is formed by the dorsal surface 

 of the pons (Fig. 482). The area thus constituted is lozenge-shaped, itg 

 widest part being opposite the superior ends of the restiform bodies or inferior 

 peduncles of the cerebellum. A thick layer of gray matter, continuous with 

 that which surrounds the central canal, is spread out like a carpet over the 

 ventricular floor, and covering this there is the usual ependymal layer which lines 

 all the ventricles of the brain. The area is circumscribed by definite lateral 

 boundaries. Thus, below, it is bounded on each side by (1) the clava, (2) tht 

 expanded upper end of the funiculus cuneatus, and (3) the restiform body ; whilst 

 above, the lateral limits are formed by the brachia conjunctiva. 



The floor of the fourth ventricle is divided into two symmetrical portions b 

 a median groove. Its lower narrow pointed portion between the two clavae receive 

 the name of the pars inferior, or, from its resemblance to the point of a pec 



Frenulum veli- -\ 



- Inferior colliculus 



- Trochlear nerve 



Velum medullare 



anterius with 



lingula 



Colliculus 

 facialis 



Area acustica 



crossed by striae - 



medul lares 



Fovea inferior -- 



Trigonum 

 hypoglossi 



Brachium conjunc 

 tivum 



Fovea superior 



. Brachium 



Brachium 

 junctivum 



" Restiform 



Stride medullares 

 --Area acustica 



- - Ala cinerea 



Funiculus separans 



- Area postrema 

 Obex 



-- Clava 



Funiculus cuneatus 





FIG. 482. FLOOR OF THE FOURTH VENTRICLE. On the right side the right half of the cerebellum has 

 removed by cutting through its three peduncles and dividing it in the median plane. On the left 

 the left half of the cerebellum is drawn over to the left so as to expose the floor of the ventricle ful 



the calamus scriptorius. Crossing each half of the floor, at its widest part, are seve; 

 more or less conspicuous bundles of fibres termed the striae medullares. They be 

 upon the lateral and posterior aspects of the restiform body, where they spri 

 from the cochlear nuclei, pass transversely medially, and disappear from view in f 

 median furrow. The striae medullares exhibit a large amount of variation 

 different specimens, both in their degree of prominence and also in the dii 

 which they pursue. It is not uncommon to find that no trace of them is visi 

 upon the surface. 



On the inferior (bulbar) district of the ventricular floor a small triangular depr 

 sion, placed immediately below the striae medullares, catches the eye. This 3 

 termed the fovea inferior. It is shaped somewhat like an arrow-head. The a] t 

 or point looks towards the striae, whilst the lateral angles of the base are prolong 1 

 downwards in the form of diverging grooves (Fig. 482). Of these, the mec 1 

 groove runs towards the opening of the central canal at the calamus scripte J, 

 whilst the lateral groove runs towards the lateral boundary of the floor. In t s 



