The Nervous System. 259 



If the divided end of the sensory nerve be irritated, that 

 portion still in communication with the spinal cord exhibits 

 signs of pain. 



In motor nerves the reverse applies. Division of a motor 

 nerve means loss of power of movement ; if that portion in 

 connection with the tissues be irritated, movement results. 

 We shall have to allude to this again in speaking of the 

 spinal cord. 



If a sensory nerve be irritated, the pain is felt in the 

 whole part supplied by that nerve and its branches ; and if 

 the motor nerve be irritated, the whole of the parts sup- 

 plied by this nerve and its branches exhibit movement. In 

 the same way division of a secretory nerve means loss of 

 secretion ; irritation of the extremity still in contact Avith 

 the gland means increased secretion. Division of a trophic 

 nerve may or may not be followed by loss of nutrition, as 

 witnessed in the comparative rarity of sloughing of the 

 foot after neurectomy. 



The effect of dividing nerves is degeneration of that 

 portion either above or below the injury. Thus division of 

 a sensory nerve above the spinal ganglion causes degenera- 

 tion of that portion still in connection with the spinal cord ; 

 whilst division of a motor nerve means degeneration of that 

 still in connection with the tissues. In this way it has 

 been determined that the nutrition of sensory nerves comes 

 from above, whilst the nutrition of motor nerves comes 

 from below the cut extremity. 



Nerves are probably nourished by the plasma which 

 reaches the axis cjdinder at Ranvier's nodes (M'Kendrick). 



The rate of transmission of impulses through human 

 motor and sensory nerves has been placed at 111 to 140 

 feet per second. 



In visceral nerves the velocity is less. Chauveau ascer- 

 tained that in the pharyngeal branches of the vagus the 

 velocity was 2G feet per second. 



Ganglia. — On certain nerves are to be found nodules 

 termed ganglia ; they are abundant in the nerves of a 

 s} 7 stem which we have yet to consider, viz., the sympathetic ; 



17—2 



