474 NERVOUS SYSTEM 



substances that have been extracted xanthin, hypoxanthin, inosite, 

 creatin and various volatile fatty acids have no special physiological 

 interest connected with the nervous system and are found in many 

 other situations. Cholesterin, which always exists in considerable 

 quantity in the nervous tissue, has been considered in connection with 

 the physiology of excretion. The ordinary 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, 

 becoming acid soon after death. 



Degeneration and Regeneration of Nerves. The degenerations 

 observed in nerves separated from the centres to which they normally 

 are 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 is divided in its course, both the motor and sensory fibres 

 of the peripheral portion undergo degeneration and lose their con- 

 ductivity. 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 integrity. 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 are divided beyond the 

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

 are exsected, the central as well as the peripheral portions degenerate. 

 These experiments show the existence of centres that 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 use- 

 ful 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 on 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 of muscles, although 

 the latter retain their contractility. This loss of excitability is gradual, 

 and it continues, whether the nerve is exposed and stimulated from time 

 to time or is left to itself, progressing from the centres to the periphery. 



