THE SPINAL CORD 671 



The position of the various groups of cells and of the above tracts 

 have been traced by various means: 



1. Wallerian degeneration (cf. p. 663) : If a nervous lesion be made, 

 such as hemisection of the cord, and the animal kept alive a sufficient 

 time for the nerve-fibre to degenerate, the path of the degenerated 

 fibres is then easily traced by certain methods of staining. Thus, 

 two or three weeks after such a lesion, degenerated fibres stain black 

 with March! 's fluid, owing to degradation of the myeline and setting 

 free of oleic acid. At a much later stage the degenerated fibres do 

 not take the Weigert-Pal stain, while normal fibres stain deeply. 

 The pathological investigation of clinical cases by this method affords 

 valuable evidence, especially in the case of the sensory tracts, where 

 the feelings, etc., of the patient have been carefully investigated. 



2. Method of retrograde degeneration: The position is traced of 

 the cells which show chroinatolysis as the result of a nervous lesion 

 e.g., section of an anterior root about three weeks before death. 

 The degeneration of different groups of cells which follows amputa- 

 tion of the leg at different levels points to a definite connection of 

 certain groups of anterior horn cells with definite muscles. 



3. Histological methods such as the silver chromate method of 

 Golgi,* or intravitam staining with methylene blue. Certain neurons, 

 \vith their processes, are picked out in their entirety. 



4. The myelination method : The Weigert-Pal method differentiates 

 the myelinated fibres from those which are not yet myelinated. The 

 development in the foetus of the medullary sheath of medullated 

 nerves takes place at different times in the various tracts. It occurs 

 first in the fibres which enter the cord from the spinal nerves; next 

 in the commissural fibres between different parts of the spinal cord; 

 then in fibres which pass from the spinal cord to the cerebellum; 

 and last in the tracts which pass from the great brain to the spinal 

 cord (the pyramids). The last become myelinated after birth. 



Tii3 Neural Arcs, We are now in a position to consider the chief 

 neural arcs through which sensory impulses are received, transmitted, 

 co-ordinated, and made effective: the spinal, cerebellar, and cerebral 

 arcs. 



The ingoing fibres, whose end processes make connection with the 

 receptors at the periphery, pass in by the posterior root, in the 

 ganglion of which their cells are situated, to form various connections 

 in the spinal cord. The chief of these are illustrated in Fig. 397, and 

 may be summarized as 



1. With the posterior horn calls of the same side. 



2. With the anterior horn cells of the same side. 



3. With the posterior horn of the opposite side. 



4. With Clarke's column of the same side. 



5. With the lateral horn of the same side. 



6. With segments of the cord higher up, and with the gracile 



* This method consists in hardening the nervous tissue in potassium chromate, 

 and soaking it in silver nitrate, thereby causing a deposition of silver chromate in 

 the norvo oell and its procc> 



