774 



A MANUAL OF PHYSIOLOGY 



nucleus of the fifth nerve a separate bundle of fibres ascends to 

 the thalamus, lying in the tegmentum of the mid-brain lateral 

 to the posterior longitudinal bundle. 



Connections of the Pyramidal Tracts. When the cortex 

 in and in front of the fissure of Rolando is destroyed by disease 

 in man, or removed by operation in animals, it is found that in a 

 short time degeneration has taken place in the fibres of the corona 

 radiata which pass off from this area. The degeneration can be 

 followed down through the genu and the anterior two-thirds of 

 the posterior limb of the internal capsule (Fig. 325) and the 

 crusta of the cerebral peduncle of the corresponding side into 

 the medulla oblongata. Below the decussation of the pyramids 



INTERNAL CAPSULE 



.Fillet 



Fillet 



CORD 



MID. BRAIN 

 FIG. 325. PYRAMIDAL PATH (AFTER GOWERS). 



Degeneration after destruction of the 'motor' area of the ri^ht cerebral 

 hemisphere. The degenerated areas are indicated by the shading. 



it is found that the degeneration has involved the two pyramidal 

 tracts, and only these the crossed pyramidal tract on the side 

 opposite the cortical lesion, the direct pyramidal tract on the 

 same side and that the cross-section of the two degenerated 

 tracts goes on continually diminishing as we pass down the cord. 

 (We overlook, for the moment, in the interest of simplicity 

 of statement, the fact that some degenerated fibres are found 

 in the crossed pyramidal tract on the same side as the lesion.) 

 This is proof positive that the cell-bodies of the neurons whose 

 axons run in these tracts are situated in the cerebral cortex. 

 They have indeed been identified with certain of the large 

 pyramidal cells (the so-called giant cells or cells of Betz) in the 

 cortex of the ' motor ' region in front of the Rolandic fissure 

 (p. 851). For after division of the motor pyramidal fibres in the 



