146 THE CEREBRUM. 



udinally by a median sulcus, the fossa inter peduncularis, and is 

 slightly concave from above downward. It is separated on either 

 side from the posterior surface by the sulcus lateralis mesencephali. 

 Though partially concealed by the temporal lobes of the cerebrum, 

 the anterior surface is unattached. It is formed by a prominent 

 band, the basis pedunculi at either side; and by a median structure, 

 the posterior perforated substance which is inclosed between the 

 two bases. The posterior perforated substance forms the floor 

 of the median sulcus. The inner border of the basis pedunculi 

 is free and overhangs the perforated substance slightly. Thus 

 is formed the oculomotor groove (sulcus nervi oculomotorii) between 

 the basis and perforated substance whence the third cerebral 

 nerve takes its apparent origin. The fourth nerve courses forward 

 over the anterior surface, but is not attached to it. 



The posterior surface (dorsal) of the mid-brain (Fig. 44), 

 though free, is entirely concealed by the cerebellar and cerebral 

 hemispheres. It forms part of the floor of the transverse fissure 

 of the cerebrum and is covered by pia mater. The lateral sulcus 

 bounds it on each side. From the sulcus lateralis it elevates 

 abruptly toward the median line, where it presents a longitudinal 

 groove. This produces two ridges which are subdivided by 

 a transverse groove into the four eminences, the colliculi of the 

 corpora quadrigemina. On either side, anterior and a little 

 external to the quadrigeminal bodies, is the medial geniculate 

 body, joined to the inferior quadrigeminal colliculus by an oblique 

 ridge, called the brachium injerius. The nearly parallel longit- 

 udinal ridges below the corpora quadrigemina are formed by 

 the brachia conjunctiva of the cerebellum. The bottom of the 

 groove between them is formed by the superior medullary velum 

 (of Vieussens), whence the trochlear nerve (fourth) is seen issuing. 



f I. Corpora quadrigemina and Brachia. 



f i. Tegmenta 

 Mid-brain < TT ~ , .. ,, , . . 



II. Pedunculi \ 2. Substantia nigra 



[ 3. Bases pedunculi. 



The four colliculi of the corpora quadrigemina and the four 

 brachia connecting them with the geniculate bodies constitute 



