292 NORMAL HISTOLOGY. 



The substantia nigra owes its dark color to the deep pigmentation 

 of its numerous nerve-cells. The latter are of medium size and of various 

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

 pyramidal ones, predominating. They enclose accumulations of dark brown 

 pigment that render the cells unusually conspicuous. The concave dorsal 

 margin of the pigmented crescent is continuous and even, but the convex 

 boundary is broken into irregular scallops by processes of gray matter that 

 penetrate the subjacent basis pedunculi. 



The basis pedunculi, the bold field that occupies the most ventral 

 part of the peduncle (Fig. 340), consists chiefly of longitudinal fibre-tracts, 

 which are passing from the cerebral cortex, by way of the internal capsule, 



3 45 



FIG. 339. Section across brain-stem (mid-brain) at level E, Fig. 329. i, substantia nigra, separating 

 basis pedunculi from tegmentum; 2, basis pedunculi ; 3, superior cerebellar peduncle and (9) its decus- 

 sation ; 4, part of pons ; 5, interpeduncular space ; 6, substantia nigra ; 7, motor tracts in basis ; 8, stratum 

 intermedium; 10, mesial fillet; n, fountain decussation ; 12, central gray substance; 13, Sylvian aque- 

 duct ; 14. inferior colliculus of corpora quadrigemina; 15, inferior brachium ; 16, posterior longitudinal 

 fasciculus ; 17, tegmental field. X 3. (Preparation by Professor Spiller.) 



to lower levels in the brain-stem and the spinal cord. The longitudinal fibres 

 are separatad into bundles by the invasion of numerous strands from the 

 fibre-complex known as the stratum intermedium, which lies along the 

 ventral border of the substantia nigra. The fibres of the basis include 

 three general sets: (i) the cortico-pontile fibres, passing from the cells of 

 the cerebral cortex to the cells of the pontile nucleus as links in the cortico- 

 cerebellar paths; (2) the cortico-bulbar fibres, passing as axones from the 

 motor neurones of the cerebral cortex to the nuclei of motor fibres origi- 

 nating within the pons and medulla; (3) the cortico- spinal fibres, passing as 

 axones from the cerebral motor neurones, through the ventral tracts of the 

 brain-stem, into the pyramidal tracts of the spinal cord, to end around the 

 radicular cells. 



The tegmentum of the mid-brain includes, as seen in transverse sec- 

 tions (Fig. 339), the area bounded by the corpora quadrigemina behind 



