778 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



and a quarter meters, or ninety feet, per second, in both motor and 

 sensory nerves, and is influenced by various conditions. Reduced 

 temperature or a great increase in temperature reduces 'the velocity of 

 nerve impulse. Anelectrotonus decreases the velocity of conduction, 

 while kathelectrotonus increases it. 



The conduction of nerve impulse is destroyed by all conditions 

 which injure the nerve, as by section, ligature, compression, or the use 

 of chemical agents which destroy its excitability at any part of its 

 course, by the removal of blood, or by the action of certain poisons, as 

 curare, which destroys the conductivity of the terminal motor-nerve 

 filaments. 



When a nerve is subjected to continued stimulation the irritability 

 of the nerve rapidly diminishes ; such a nerve is said to be exhausted or 

 to be in a condition of fatigue. A nerve in which exhaustion has 

 occurred may again regain its activity, provided the stimulation has not 

 been too excessive or too greatly prolonged. 



In cold-blooded animals stimulation may be much more severe and 

 protracted without producing exhaustion than in warm-blooded animals, 

 and, while nerves are more slowly affected than muscles, the recovery of 

 the former is more slowly accomplished than in the latter. 



When a nerve-fibre is separated by section from the central nervous 

 system the condition of the nerve will vary according as to the function 

 of the nerve. Provided a nerve be in connection with the nerve-centres 

 which govern its nutritive processes, it may be divided at any part of its 

 course and degeneration of the nerve will only occur in those parts which 

 have been separated from the nutritive centre. Thus, for example, if a 

 motor nerve be divided the peripheral extremity of the nerve will 

 become disorganized, while the part still in connection with the spinal 

 cord will remain intact. 



If a purely sensory nerve be divided it would at first appear that 

 the same condition prevailed ; and if, again, a mixed nerve be divided 

 the peripheral part of the nerve, including all its branches, will degenerate, 

 while the central parts will remain intact. 



The centres governing the nutrition of motor and sensory nerves 

 are not, however, as might appear from the above statements, the same. 



If the anterior root of a spinal nerve be divided before it joins the 

 posterior root the motor fibres in the spinal nerve formed by the union 

 of this anterior and posterior root will degenerate, while the portion 

 remaining in connection with the cord will remain intact. If the pos- 

 terior root be divided between the spinal cord and its ganglion the part 

 of the nerve lying between the point of division and the spinal cord will 

 degenerate, while the peripheral portions of the part between the point 

 of section and the posterior ganglion will remain intact. This indicates 



