THE RHOMBENCEPHALON 



657 



inclines dorsally, and opens in the posterior angle of the fourth ventricle. Hence it 

 is customary to distinguish a closed and an open part of the medulla. The dorsal 

 aspect of the latter, which is concealed at present, will be considered later in the 

 description of the fourth ventricle. 



The lateral surface is narrow behind, wider and rounded in front. From it 

 the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear 

 series, and alongside of it the spinal part of the eleventh nerve passes forward to 

 join the medullary root. Close inspection reveals the presence of strige which 

 curve obliquely downward and backward from the surface of the restiform body 



LoiKjiliuliual fissure 



Sulcus rl)inalis~. 



Optic citidsitxi^ 



Tuber cinereuin'-^ 

 PyriJ'onn lubc-^ 



Piluilanj budy- 



III- 



( sensory root - 



v\ 



I motor root -^ 



Cerebellum __^ 



Tuberculum faciale 



Olfactory bulb 



__, Olfdclory peduncle 



___,- I iitcruid olfactory tract 



, Extcnad olfactory tract 



— TrigoHuin olfuctorium 

 ^\~~ Fossa transversa 



- Cerebral peduncle 



- Tractus pcd. iransrersus 

 ^_ Interpeduncular space 



Pons 



Corpus trapezoideum 



Pyramid 



Medulla 



~"~~~- Chorioid plexus of fourth 



,_^ J ventricle 



^\ Median fissure and de- 



cussation of pyramids 



: — Spinal cord 

 Fig. 500. — Line Dr.\wing of B.\se of Br.\i.\ of Horse. (Key to Fig. 499.) 



toward the hypoglossal root-fibers; these are the external arcuate fibers (Fibrse 

 arcuatae externae). The recess between the lateral aspect of the medulla and the 

 cerebellum is occupied by an irregular mass of villous projections of the pia mater, 

 containing tufts of vessels; this is the chorioid plexus of the fourth ventricle, and 

 is the lateral edge of the tela chorioidea of the ventricle. On raising the chorioid 

 plexus it is seen that the tela chorioidea is attached to the dorsal aspect of the 

 medulla, and reinforces here the wall of the fourth ventricle ; also that the restiform 

 body terminates in front by entering the base of the cerebellum, forming its pos- 

 terior peduncle. 



In the medulla the fiber tracts of the spinal cord either terminate in the nuclei of the gray- 

 matter or undergo changes in their relative position, and new tracts appear. The gray matter 

 is highly modified and forms masses which have no homologues in the cord. The central canal 

 of the closed part of the medulla is surrounded by a thicker layer of gray matter than is the case 

 in the spinal cord. As the medulla opens out this gray matter is naturally spread in the floor of 



