BASE OF THE BRAIN. 191 



corpora albicantia (e) ; and anterior to these a grayish mass, called tuber 

 cinereum (//). From the tuber cinereum a conical reddish tube, the in- 

 fundibulum, descends to the pituitary body in the sella Turcica of the 

 sphenoid bone. 



Anterior to the tuber cinereum are the converging optic tracts with 

 their commissure (/). Beneath the commissure lies a thin grayish layer 

 (lamina cinerea, M) ; and still further forwards is the great longitudinal 

 fissure between the hemispheres, with the white corpus callosum (n) in 

 the bottom of it. 



At the inner end of the transverse fissure (Sylvian) across the hemi- 

 sphere, is another spot, perforated by vessels, and distinguished by the 

 name substantia perforate, or locus perforatus anticus (/). 



Peduncle of the cerebrum (crus cerebri, /). This is a large, white, 

 stalk-like piece, which reaches from the upper border of the pbns to the 

 under part of the cerebral hemisphere of the same side, near the inner 

 margin. In the natural position the two peduncles fill the opening in the 

 tentorium cerebelli. Each is about three-quarters of an inch long, and 

 widens as it approaches the cerebrum. Crossing its outer surface is the 

 optic tract ; and between the crura of opposite sides is the interpeduncular 

 space, which contains the locus perforatus, the corpora albicantia, and the 

 tuber cinereum. 



Structure. The peduncle consists of longitudinal fibres, continuous with 

 the longitudinal fibres of the pons, which inclose here a mass of gray mat- 

 ter between them. 



Dissection. For the purpose of showing the structure of the crus, on 

 the right side (fig. 57), the optic tract should be divided, and the fibres 

 continuous with the anterior pyramid of the medulla oblongata should be 

 raised as far into the crus as the optic thalamus. In this proceeding 

 the mass of gray matter (locus niger) will appear, and beneath it will be 

 seen a second or deeper set of longitudinal fibres. 



The superficial fibres, which form the under or free part of the crus, 

 are continued from the anterior pyramidal body (fig. 06, c). They are 

 longitudinal in direction, and coarse in texture, and are directed upwards 

 radiating to the cerebrum. The surface of the peduncle, which is com- 

 posed of these fibres, is called the fasciculated portion, or the crust. 



The deeper fibres are also prolonged to the cerebrum. They are derived 

 chiefly from the lateral tract and posterior pyramid of the medulla oblon- 

 gata, with a slip from the olivary fasciculus (fig. 57, d). Others come 

 from the cerebellum, and mix with the former: some of these decussate 

 across the middle line 1 (p. 215). The fibres obtained from these four 

 sources are situate beneatli (as now seen) the gray matter : besides being 

 deeper they are finer than the superficial set, and inclose much gray sub- 

 stance: the part of the crus formed by them is named tegmentum. 



The gray matter (locus niger) of the crus (fig. 57, #) forms a thin layer, 

 which reaches nearer the inner than the outer margin of that body ; it is 

 convex towards the free surface, but concave in the opposite direction. 



The posterior perforated spot (pons Tarini, g) is situate between the 

 peduncles of the cerebrum ; in the area of the space is a layer of grayish 



1 M. Foville describes a median commissure for the medulla oblongata, pons, 

 and cerebral peduncles, which is composed of the interweaving of fibres of oppo- 

 site sides. See the work of M. Foville, entitled : TraiM complet de V Anatomic, $r., 

 du Syst&me Nerveux certbro-spinal, p. 323, 1844. 



