THE MESENCEPHALON. 



1109 



cross-sections, (Fig. 960) its outline in a general way is triangular, with the base 

 above and the apex directly below ; but the contour of the canal varies at different 

 levels, being triangular near its extremities and irregularly cordiform or elliptical in 

 the intervening part of its course. 



INTERNAL STRUCTURE OF THE MESENCEPHALON. 



Disregarding the several small nuclei, the nuclei of the corpora quadrigemina 

 and the red nuclei, the gray matter within the mesencephalon is disposed as three 

 tracts that extend the entire length of the mid-brain. 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 portions. 



The central gray matter (stratum griseum centrale) completely encloses 

 the cavity of the mid-brain and hence is often called the Sylvian gray matter. It 

 contains numerous irregularly scattered nerve-cells of uncertain form and size, 

 and, along its ventral border, the nuclei of origin 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. 



FIG. 960. 



Inferior colliculus 



Mesencephalic 

 root of trigeminus 



Lateral fillet 

 Fibres of fourth nerve 



Nucleus of 

 fourth nerve 



Mesial fillet 



Sylvian aqueduct 



Central gray substance 



Posterior longitudinal 

 fasciculus 



uiitain decussation 



Mesial fillet 



Cerebellar peduncle 



Decussation of cerebellar peduncle 



Transverse section of dorsal part of mid-brain through lower end of inferior colliculi, at level M (Fig. 919) 

 showing nucleus of trochlear nerve, and decussation of cerebellar peduncle. Weigert-Pal staining. X 3&. 

 Preparation by Professor Spiller. 



The substantia nigra is disposed as two irregular crescentic columns of dark 

 gray matter that separate the tegmentum from the crustae of the peduncles. The 

 substance begins below at the upper border of the pons and continues uninterruptedly 

 through the length of the mid-brain into the subthalamic region of the diencephalon, 

 where it gradually disappears. The deep color of this tract is due to the conspicuous 

 pigmentation of its numerous nerve-cells. These cells are of medium size and of 

 various form, spindle-shaped elements, interspersed with some of stellate and a few 

 of pyramidal form, predominating. They enclose considerable accumulations of dark 

 brown pigment that render the cells unusually conspicuous. During the earliest 

 years of childhood the pigmentation is absent or very slight, but after the sixth 

 year it is marked, and by the seventeenth has acquired its full intensity. Seen in 

 cross-sections (Fig. 961), the convexity of each column, directed forward and out- 

 ward, is not uniform, but broken into irregular scallops by processes of gray matter 

 that penetrate the subjacent crusta. The concave dorsal margin, on the contrary, is 

 unbroken and even. The horns of the crescentic areas, of which the median is 

 somewhat the thicker, approach the free surface along the bottom of the superficial 



