THE CENTRAL NERVOUS SYSTEM 775 



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) completely 

 and permanently, without entailing death for a considerable 

 time (Holmes and May) . The fact that after destruction of the 

 cortex or the path in its course the degeneration below the lesion 

 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 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) As already mentioned (p. 756), comparatively transient but 

 decided changes occur in the anterior horn cells on section of the 

 corresponding anterior roots. 



(4) 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 (about 

 80,000 in each half of the human cord) falls far short of the number 

 of fibres in the anterior roots (not less than 200,000 in man on each 

 side) , it is necessary to suppose either that one pyramidal fibre may 

 be connected with several cells or that all the anterior root-fibres 

 are not in functional connection with the pyramidal tract. 



There is no reason to assume any such connection in the case of 

 the fine medullated root -fibres arising in the lateral horn and going 

 to the visceral and vascular muscles. 



While there is no doubt that anterior root-fibres and pyramidal 

 fibres of the brain and cord form segments of the same nervous 

 path, the connection between the pyramidal fibres and the cells 

 of the anterior horn has not yet been anatomically demonstrated. 

 Many of the pyramidal fibres pass into the grey matter between 

 the anterior and posterior horns or near the base of the posterior 

 horn. The anterior horn cells are surrounded by arborizations. 

 Some of these are probably the terminations of axons whose 

 cell-bodies are situated in the posterior horn, others the termina- 

 tions of posterior root- fibres or their collaterals. Many of them 

 very likely represent the end arborizations of pyramidal fibres 



* Such enumerations can be made with great accuracy from photographs 

 of sections of the nerves (Hardesty, Dale). (See Fig. 312, p. 758.) 



