304 NORMAL HISTOLOGY. 



of the medulla and the pons. On transverse section of the crura, 

 it is seen that the tegmental halves are united, while the two 

 peduncular portions are widely separated and are attached to 

 the tegmentum alone ; the oblique line of this juncture is indicated 

 within the section by a deeply pigmented area, the substantia 

 nigra. 



The crusta is hemi-cyliridrical in section, but the encroachment 

 of the substantia nigra reduces the area devoted to the ascending and 

 descending fibres to a narrow crescent, whose convexity corre- 

 sponds to the external outline of the peduncle, while the concavity 

 embraces the dark field. Since the tracts of ventral nerve-fibres of 

 the peduncle greatly exceed the pyramidal bundles of the pons, it is 

 evident that many additional fibres have arisen within the peduncles. 

 On reaching the cerebral hemispheres in their course upward, the 

 tracts of the crusta become continuous with the fibres constituting 

 the internal capsule. 



The substantia nigra, separating the crusta and the tegmentum, 

 forms a tract of gray matter extending from the upper border of the 

 pons forward as far as the mammillary bodies ; while it gradually 

 diminishes in its forward course, the mesial edge of the mass becomes 

 thickened in the vicinity of the oculo-motor groove. The area owes 

 its exceptional color to irregular groups of deeply pigmented 

 multipolar cells embedded within a finely granular ground-sub- 

 stance. 



The tegmentum forms only part of the great nuclear tract 

 continued through the dorsal portion of the oblongata, the pons, and 

 the peduncle into the subthalamic region ; as in the other localities, 

 so here, the stratum of gray matter lying beneath the floor of the 

 neural tube and the formatio reticularis are its principal constit- 

 uents. In addition to the gray matter distributed throughout the 

 reticulum, groups of nerve-cells are situated along the floor of the 

 Sylvian aqueduct ; some of these are of importance as the nuclei 

 of the bundles of the oculo-motor and the pathetic nerve. Near the 

 middle of the formatio reticularis, on either side of the raphe", lies a 

 conspicuous group of large pigmented nerve-cells, the tegmental or 

 red nucleus, so called on account of its brown or reddish hue. The 

 formatio reticularis of the tegmentum differs little from the similar 

 structure at lower levels. In general, the fibres contained within 

 the crusta connect with the motor areas of the cerebral cortex, 

 while those of the tegmentum usually terminate in or about the 

 thalamus. 



THE CEREBELLUM. 



The cerebellum consists of a peripheral or cortical layer of gray 

 substance which encloses the various tracts of nerve-fibres composing 



