THE NERVOUS SYSTEM 427 



staining of normal and degenerated fibres. Cholin, another 

 product of the decomposition of lecithin, is very poisonous. 

 Cerebrosides are a group of glucoside bodies containing nitrogen, 

 of which very little is known, excepting that they yield galactose 

 on decomposition. 



Irritability and Conductivity. 



Two features of nerve-fibres are irritability and conductivity. 

 By irritability is understood the reaction to a stimulus, by con- 

 ductivity the propagation of an excitation. The nature of the 

 change occurring in a centre or receptive surface which induces 

 an impulse is as yet unknown, but whatever the change may 

 be it is experimentally possible to imitate it artificially by 

 means of chemical, mechanical, or electrical stimuli, and when 

 nerves are so stimulated they function as if the normal stimulus had 

 been applied. Electrical stimuli are most commonly employed, 

 and if a motor nerve be so stimulated the muscle contracts ; if a 

 sensory nerve, pain or a sensory impression is observed. If a 

 secretory nerve be stimulated, secretion results. At one time the 

 normal stimulus to a nerve-cell was believed to be electrical in 

 nature ; it is now known that this is not so, and that though 

 electrical stimuli are capable of causing a nerve to function, it is 

 a coarse method compared with the natural stimulus. A sudden 

 change in temperature acts as a stimulus to a sensory, but not 

 to a motor nerve. A motor nerve is quite unaffected by sudden 

 cooling or heating, whereas a sensory nerve so treated produces 

 pain, and it has been suggested that this difference in reaction 

 indicates some marked difference in structure. When nerves are 

 ligatured or divided impulses are no longer transmitted. Ex- 

 perimental inquiry shows that even after long-continued excita- 

 tion nerves are still irritable, which has given rise to the belief 

 that nerves never suffer from fatigue (see p. 432). 



When dealing with the question of muscular contraction 

 (p. 395), it was stated that the stimulation of a nerve-muscle 

 preparation by means of a constant current caused a ' twitch ' 

 of the muscle at ' make,' and another at ' break/ and that during 

 the period the current was passing through the nerve, though the 

 muscle gave no evidence of this, yet important changes were 

 occurring. These changes are concerned with the irritability and 

 conductivity of the nerve, and must here be examined. 



Electric Phenomena in Nerves. — When studying a muscular con- 

 traction, we saw that a perfectly uninjured muscle was iso-electric 

 — viz., it gives no evidence to the galvanometer of the existence of 

 a current. If, however, the muscle be injured, the seat of injury 

 became electrically negative to the uninjured part, and the current 

 was called the current of injury. The figure there employed to 



