DEGENERATION AND REGENERATION OF NERVES. 521 



ine, which always exists in considerable quantity in the nervous tissue, has 

 been considered in connection with the physiology of excretion. The ordi- 

 nary fats are in combination with other fats or with peculiar acid substances, 

 The reaction of nerve-tissue is either neutral or faintly alkaline under normal 

 conditions, soon becoming acid after death. 



Degeneration and Regeneration of Nerves. The degenerations observed 

 in nerves separated from the centres to which they are normally attached, 

 first studied by Waller, in 1850, are now used in following out certain 

 nervous connections too intricate to be revealed by ordinary dissection. 

 This is known as the Wallerian method. If an ordinary mixed nerve be 

 divided in its course, both the motor and sensory fibres of the peripheral 

 portion undergo fatty degeneration and lose their excitability. As regards 

 the spinal nerves, degeneration occurs in the motor fibres only, when the 

 anterior spinal root has been divided, and the nerve has degenerated fibres 

 (motor) mixed with the sensory fibres, which latter retain their anatomical 

 and physiological characters. The motor fibres of the spinal nerves are 

 degenerated when separated from their connections with the anterior cornua 

 of gray matter of the cord. If the posterior roots of the spinal nerves be 

 divided beyond the ganglia, the peripheral sensory fibres degenerate ; but if 

 the ganglia be exsected, the central as well as the peripheral portions degen- 

 erate. These experiments show the existence of centres which preside over 

 the nutrition of the nerves. The centres for the motor filaments of the 

 spinal nerves are in the anterior cornua of gray matter of the cord. The 

 centres for the sensory fibres are the ganglia of the posterior roots. The 

 centres for the sensory cranial nerves are the ganglia on their roots ; and 

 the centres for the motor cranial nerves are probably the gray nuclei of 

 origin of these nerves. The Wallerian method has been found useful in 

 studying the paths of conduction in the encephalon and spinal cord, as will 

 be seen in connection with the physiology of these parts. 



The excitability of the motor nerves disappears in about four days after 

 their section. Of course, in experiments upon this point, it is necessary to 

 excise a portion of the nerve to prevent reunion of the divided extremities ; 

 but when this is done, after about the fourth day, stimulation of the nerve 

 will produce no contraction in the muscles, although the latter retain their 

 contractility. This loss of excitability is gradual, and it continues, whether 

 the nerve be exposed and stimulated from time to time or be left to itself, 

 progressing from the centres to the periphery. In the researches of Longet 

 upon this subject, it was found that the lower portion of the peduncles of the 

 brain lost their excitability first, then the anterior columns of the cord, then 

 the motor roots of the nerves, and last of all, the branches of the nerves near 

 their terminations in the muscles. 



The sensibility of the sensory nerves disappears from the periphery to 

 the centres, as is shown in dying animals and in experiments with anaesthet- 

 ics. The sensibility is lost, first in the terminal branches of the nerves, next 

 in the trunks and in the posterior roots of the spinal nerves, and so on to the 

 centres* 



