144 THE CEREBRUM. 



and is joined to the inferior quadrigeminal eminence by the 

 brachium inferius. The brachium superius sweeps around it in 

 front. The medial geniculate body is gray in color and is not 

 laminated. Its cell-bodies are small, and fusiform in shape. 

 They perhaps give origin to the intercerebral fibers (Guddeni) 

 of the optic tract and to a large part of the acustic radiation. 



CEREBRUM. 



SECTION II. THE MID-BRAIN. 

 (MESENCEPHALON.) 



The third division of the cerebrum is the mid-brain (Figs. 

 44 and 45). It is situated below and behind the inter-brain and 

 forms the connecting link between the fore-brain and the hind- 

 brain. This has suggested the name "isthmus," sometimes 

 applied to it: though isthmus rhomb encephali refers only to the 

 constriction below the corpora quadrigemina. The mid-brain 

 is developed from the middle of the brain vesicles, the mesencepha- 

 lon (Figs. 16, 17 and 18). The cavity of the mesencephalon 

 persists as the cerebral aqueduct, which is reduced to a slender 

 canal by the thickening of its walls, roof and floor, due largely 

 to the ingrowth of fibers from other parts of the brain. The 

 cerebral hemispheres almost conceal the mid-brain from view; 

 they overhang it dorsally, and the temporal lobes, inclosing it 

 between them, bend medially and cover part of its anterior surface. 

 Only the median part of the anterior surface is visible in the 

 complete brain (Fig. 21). The form of the mid-brain resembles 

 a flattened cylinder. Its axis, a half inch long, is pointed upward 

 and forward, and its long diameter, which varies from an inch 

 to an inch and a half in length, is directed transversely. 



SURFACES. 



The mid-brain has four surfaces, viz., the anterior and posterior, 

 which are free, and the superior and inferior, representing the 

 ends of the cylinder, which are attached. The two latter are 

 parallel with each other and are formed by section. 



The superior surface, sloping downward and forward, meets 



