THE CEREBRAL PEDUNCLES. 291 



only the tegmental but also the ventral part of the pons to gain its lower 

 border, along which they emerge. Another nucleus of the middle area, the 

 superior olivary nucleus, lies near the ventral limit of the tegmental area, 

 partly lodged within an indentation on the dorsal aspect of the conspicuous 

 tract of transverse fibres, the corpus trapezoides. This nucleus, often called 

 the superior olive, is an irregular spherical collection of neurones interposed 

 in the path connecting the auditory nuclei with the cerebral cortex and 

 closely related with the tract of the lateral fillet. A small collection of 

 nerve-cells between the fibres of the trapezoidal tract constitutes the nucleus 

 trapezoides. The facial nucleus is a broken mass of gray matter that 

 includes several groups of large stellate motor neurones lying to the inner 

 side of the emerging facial root-fibres. 



The ventral part of the inner area and the adjoining portion of the 

 middle one are occupied by the field of the median fillet, which, at the level 

 under consideration, no longer lies with its long axis directed dorso-ventrally, 

 but approximately horizontal (Fig. 336). The tract now appears as a com- 

 pressed and modified oval, with the thicker inner end close to the raphe and 

 the tapering outer one near the superior olive. The posterior longitudinal 

 fasciculus is seen as a compact strand, immediately beneath the gray matter 

 of the ventricular floor and at the side of the raphe. Within the outer 

 area, lateral to the facial fibres, appear the substantia gelatinosa and the 

 associated descending root of the trigeminal nerve. Just dorsal to the latter, 

 the descending vestibular root lies close to the inner side of the restiform body. 



THE CEREBRAL PEDUNCLES. 



V 



That part of the brain-stem which encloses the Sylvian aqueduct corre- 

 sponds approximately with the morphological division of the brain known 

 as the mesencephalon. The latter includes two main subdivisions : ( i ) the 

 smaller dorsal part, the quadrigeminal plate, which roofs the Sylvian aque- 

 duct and bears the corpora quadrigemina, and (2) the much larger ventral 

 part made up by the cerebral peduncles. 



The latter, also called the cerebral crura, are fused dorsally into a con- 

 tinuous tract, the tegmentum, which contributes the side-walls and floor of 

 the Sylvian aqueduct and blends with the overlying quadrigeminal plate. 

 Ventrally the peduncles are unfused and appear on the inferior surface of 

 the brain (Fig. 327) as two robust diverging stalks, enclosing the lower 

 half of the interpeduncular space. In transverse sections (Fig. 339), each 

 stalk is seen to include a ventral zone of cross-cut nerve fibres, the basis 

 pedunculi, and a crescentic area of deeply pigmented gray matter, the sub- 

 stantia nigra, that separates the basis pedunculi from the tegmentum. 



Disregarding the several small nuclei, the nuclei of the corpora quadri- 

 gemina and the red nuclei, the gray matter within the mesencephalon is 

 disposed as three tracts that extend its entire length. These are the tubular 

 mass of the central gray matter, which surrounds the aqueduct, and the two 

 crescentic columns of the substantia nigra, which subdivide the peduncles 

 into the tegmental and basal parts. 



The central gray matter {stratum griseum ccntrale} encloses the 

 Sylvian aqueduct and contains numerous irregularly scattered nerve- 

 cells of uncertain form and size and, along its ventral border, the nuclei of 

 the oculomotor and trochlear nerves ; within its lateral parts lie the nuclei 

 from which proceed the fibres of the mesencephalic roots of the trigeminal 

 nerves. 



