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A TEXTBOOK OF PHYSIOLOGY 



retrograde degeneration occurs, the nerve cells connected with par- 

 ticular nerve-fibres can be traced. It has been shown, moreover, 

 that such a degeneration may occur in other cells closely connected 

 with the severed neuron; for example, cutting the posterior roots of 

 a nerve causes chromatolysis in the closely connected cells of the 

 anterior horn of the spinal cord. 



The nutritive function of the nerve cell is most clearly seen in 

 the axon after its section. When an axon is cut, the part of the axon 

 cut off from the nerve cell undergoes Wallerian or primary degen- 

 eration. 



The Effects of Section of the Axon. In the first place, the conduc- 

 tion of impulses is interrupted. A current of injury is set up in the 

 nerve which soon subsides, and is followed by a gradual loss of 

 excitability and conductivity; the loss begins in the most central 

 part of the cut peripheral end, and gradually passes towards the 

 periphery. 



FIG. 316. 



A shows pericellular chromatolysis of anterior horn cell; B shows perinuclear chro- 

 matolysis; C shows degeneration with vacuolation of the cytoplasm. (Mott.) 



Following upon this, Wallerian degeneration occurs. The complex 

 lipoid material of the medullated coat splits up, and the lecithin it 

 contains breaks down, yielding cholin and fatty acid. The staining 

 properties of the myelin change, owing to the setting free of oleic 

 acid, and thus the degenerate fibres, two or three weeks after their 

 section, stairi black with Marchi's fluid. Normal fibres do not stain 

 so. By this staining reaction it is possible to trace the course of 

 degenerating fibres in, and so unravel the structure of, the central 

 nervous system. The myelin breaks up into droplets, and finally is 

 absorbed. The nuclei multiply in the fibre, and finally the axon goes 

 and the nucleated sheath is alone left. Old degenerated fibres are 

 marked by their failure to stain \vith the Weigert-Pal method. 



Regeneration of Nerve. Even during the degeneration of the axon 

 the first regenerative change begins to take place. This consists of 



