THE PONS AND THE CEEEBELLUM. 319 



pass separately the lingula, lobulus centralis, uvula, and nodulus. A 

 number (five) of the lobules of the monticulus are united with the vertical! 

 branch {Vert. A) before their juncture with the main truck of the arbor 

 vit£e. The posterior portion of the monticulus, together with the folium 

 eacuminis and the tuber valvulffi, unite to form the horizontal Iranch {Horiz. 

 A) of the arbor vitse. 



Posteriorly the velum medullare postieum passes from the cerebellum 

 as roof of the fossa rhomboidalis as far as to the termination of the posterior 

 column of the spinal cord. This roof consists only in its lateral portions of 

 rather dense tissue, principally neuroglia; in the median line it is simply a 

 layer of cuboidal epithelium. From the pia numerous vascular loops pass 

 into this membrane and press it somewhat into the ventricle (plexus 

 chorioideus ventriculi quarti, or plex. chor. medialis). But the most later- 





p3;orpt,-dp Horiz.A^ 



Pig. 206. — Sagittal section through the middle of the Vermis. 



ally located portions of the membrane form, in the region of the oblongata, 

 sack-like projections whose median wall, through vascular loops, likewise 

 becomes a choroid plexus: plexus chorioideus lateralis (see Fig. 243); In 

 the median as well as in the place of origin of the lateral plexus perforations 

 are found (Key and Betzius). The middle one of these openings is called the 

 foramen of Majendie. It is of great importance in the ready equalization 

 and adjustment of variations of pressure in the cerebro-spinal fluid. 



Into the cerebellum pass the three pairs of commissural arms. They 

 pass into the central white substance and form connections there with gray 

 nuclei; they also send fasciculi to the cortex of the cerebellum. 



In the cortex one may even with low magnification differentiate three 

 different layers: Externally lies the zona molecularis; internally the zona 

 granulosa; while between them one finds a layer of very large cells — the 

 cells of Purhinje. 



