CONNECTIONS OF THE LONG PATHS OF THE CORD 847 



Fillet 



in the fillet. Many of them occupy a position in the reticular forma- 

 tion of the tegmentum dorsal to the fillet as they pass through the 

 pons and mid-brain to end in the thalamus and the region below it 

 (sub-thalamic region). From the sensory nucleus of the fifth nerve 

 a separate bundle of fibres ascends to the thalamus, in the tegmen- 

 tum 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- 343) and the 

 crusta of the cerebral 

 peduncle of the corre- 

 sponding side into the 

 medulla ojblongata. 

 Below the decussation 

 of the pyramids it is 

 found that the degene- 

 ration has involved the 

 two pyramidal tracts, 

 and only these the 

 crossed pyramidal 

 tract on the side oppo- 

 site the cortical lesion, 

 the direct pyramidal 

 tract on the same side 

 and that the cross- 

 section of the two de- 

 generated tracts goes 

 on continually dimin- 

 ishing 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. 918). For after division 

 of the motor pyramidal fibres in the upper cervical region of the cord 

 (in monkeys) changes in the chromatin (so-called chromatolysis) and 

 atrophy of these large cells occur. The same has been found to be 

 true in man in cases where the cord was injured by fracture of the 

 spine in such a way as to interrupt the tract (as well as other tracts) 



INTERNAL 



CORD 



MID. BRAIN 



Fig. 343. Pyramidal Path (after Cowers). Degenera- 

 tion after destruction of the ' motor ' area of the 

 right cerebral hemisphere. The degenerated areas 

 are indicated by the shading. 



