H38 



A MANUAL OF ANATOMY 



medulla oblongata. In the lower part of the pyrcimid they give 

 rise to the crossed and direct pyramidal tracts. The crossed 

 pyramidal tract, having taken part in the decussation of the 

 pyramids, descends in the spinal cord, occupying the posterior 

 part of the lateral column of the opposite side. The direct pyra- 

 midal tract descends in the spinal cord, occupying the inner portion 

 of the anterior column of the same side, and forming the column 

 of Tiirck. Its fibres, however, cross at intervals to the opposite side. 



As the pyramidal tract descends through the pons Varolii and 

 meduUa oblongata, some of its fibres pass to the motor nuclei of 

 the cranial nerves in these regions. 



The cortico-pontine or cortico-protuberantial fibres are arranged 

 in two strands, fronto-pcntine and temporo-pontine. The fibres 



Aqueduct of Sylvius, 



_A Corpora Quadrigemina 



Grey Matter around Aqueduct 

 of Sylvius 



Red Nucleus of Tegmentum 



Temporo-Pontine Fibres 



—Locus Niger 



_ Pyramidal Fibres 

 -.>^ Fronto Pontine Fibres 



Oculo-Motor Sulcus 

 Fig. 603. — Transverse Section of the Crus Cerebri (after Poirier). 



of the fronto-pontine strand arise from the cells of the cortex of 

 the anterior part of the frontal lobe, and, having traversed the 

 anterior limb of the internal capsule, they are regarded as occupying 

 the inner fifth of the crusta. The fibres of the temporo-pontine 

 strand arise from the cells of the cortex of the temporal lobe, and, 

 having traversed the postlenticular part of the internal capsule, 

 they occupy the outer fifth of the crusta. In the ventral part of 

 the pons both the fronto-pontine and the temporo-pontine fibres 

 terminate in arborizations around the cells of the nucleus pontis, 

 whereas the pyramidal fibres pass uninterruptedly through the 

 ventral part of the pons. 



Summary of the Crusta. — The crusta is divided into three parts. The 

 middle three-fifths contain the fibres of the pyramidal or motor tract, the 

 inner fifth probably contains the fibres of the fronto-pontine strand, and the 

 outer fifth contains the fibres of the temporo-pontine strand. 



Substantia Nigra. — This is a mass of dark grey matter which is 

 situated between the tegmentimi and the crusta of the crus cerebri. 

 IJke the crusta, it is semilunar or crescentic, as seen in transverse 

 section. It contains many multipolar nerve-cells, which are deeply 

 pigmented, and it extends from the upper border of the pons 



