6S6 A MANUAL OF PHYSIOLOGY 



two-thirds of the posterior limb of the internal capsule 

 (Fig. 250), and the crusta of the cerebral peduncle of the 

 corresponding side into the medulla oblongata. Below the 

 decussation of the pyramids 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 in the monkey and in man, at any 

 rate, some degenerated fibres may be 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 pyra- 

 midal cells in the Rolandic area. Among these the so-called 

 giant-cells, which are most numerous in the upper portions 

 of this area, and the majority of which are connected with 

 the long pyramidal fibres that run down into the cord, are 

 especially conspicuous. The fact that the degeneration does 

 not spread to the anterior roots shows that at least one 

 relay of nerve-cells intervenes between the pyramidal fibres 

 and the root-fibres. The results both of normal and morbid 

 histology enable us to identify the cells of the anterior horn 

 as the cells of origin of the axons of the anterior root-fibres. 

 For 



(1) Axis-cylinder processes have been actually observed passing 

 out from certain of the so-called motor cells of the anterior horn to 

 become the axis-cylinders of fibres of the anterior root. 



(2) In the pathological condition known as anterior poliomyelitis, 

 the cells of the anterior horn degenerate, and so do the anterior 

 roots of the affected region, the motor fibres of the spinal nerves, 

 and the muscles supplied by them. 



(3) An enumeration* has been made in a small animal (frog) of 

 the cells of the anterior horn and of the anterior root-fibres, and it has 

 been found that the numbers agree in a remarkable manner. From all 

 this it cannot be doubted that most, at any rate, of the cells of the 

 anterior horn are connected with fibres of the anterior root. But 

 since the number of fibres in the pyramidal tracts falls far short of 



* Such enumerations can now be made with great accuracy from 

 photographs of sections of the nerves (Hardesty, Dale). (See Fig. 236, 

 p. 669.) 



