66 Anato:my of the Nervous System 



fibres, and Edinger believed it to be composed largely of 

 fibres belonging to the ciliary portion of the oculomotor. It 

 runs along the medial edge of the pes pedunculi and then 

 turns dorsally a short distance to end in a centre close to the 

 substantia nigra and posteroventral to the red nucleus 

 {nucleus tractits pedunculi transversus, ganglion ectomamillare 

 of Wallenberg). Kosaka and Hiraiwa found that removal 

 of the eye of the rat produced degeneration of this tract, 

 chiefly contra- but partly homo-laterally. Some of the fibres 

 enter the subthalamic nucleus. The secondary fibres from 

 the nucleus of the transverse peduncular tract are probably 

 short, running forward into the subthalamic nucleus for 

 reflexes of the pupil, eyelid, etc. 



Just in front of the level of the anterior border of the pons, 

 there appears a mass of gray matter separating the teg- 

 mentum from the ventrally situated basis pedunculi. This 

 mass is the substantia nigra, which has the form of a broad, 

 thick band extending forward into the hypothalamus, and 

 becoming thicker and more rounded in cross section as it 

 does so (Pls.XI\^-XYI.). It receives fibres from the cerebral 

 hemispheres and gives rise to others which run through the 

 tegmentum, but its precise connections and function are 

 not clearly understood. It has received its name on account 

 of its dark colour in the human brain, where it stands out 

 conspicuously when examined in gross material. This colour 

 is due to the presence in many of the cell-bodies of large 

 numbers of brown pigment granules. There is less pigmenta- 

 tion in lower mammals than in higher ones, however, (Kap- 

 pers) and in the rat it is practically entirely absent. 



The basis pedunculi is composed of a broad band of closely- 

 grouped, longitudinal fibres running along each side of the 

 ventral surface of the midbrain, and appearing roughly 

 crescentic in cross section (Pis. II., XIY.-XIX.). The fibres 

 originate in the cerebral cortex and pass through the internal 

 capsules to form these bands, which converge towards the 



