876 THE CENTRAL NERVOUS SYSTEM 



completely and permanently, without entailing death for a consider- 

 able 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 



(i) 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. 858), comparatively transient but 

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

 sponding 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 80,006 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 liorn 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 terminations of posterior root-fibres 

 or their collaterals. Many of them very likely represent the end 

 arborizations of pyramidal fibres or their collaterals. Some ob- 

 servers, however, suppose that the pyramidal fibres do not come into 

 immediate relation with the anterior horn cells, but that another 

 neuron is intercalated between them and the cells. 



The pyramidal fibres are unquestionably paths for voluntary 

 motor impulses passing down from the cortex to the cord. But 



* Such enumerations can be made with great accuracy from photographs 

 of sections of the nerves (Hardesty, Dale). (See Fig. 341, p. 860.) 



