362 



THE NERVOUS SYSTEM. 



FIG. 95. 



with the amputated limb (Fig. 95). If the electrodes be now applied 

 to the exposed nerve, and a current allowed to pass between them, 

 at the moment of its passage a contraction takes place in the muscles 

 below. In this case the electric current acts on the nerve 

 alone: and the nerve excites the muscles by its own 

 special agency. A muscular contraction, therefore, under 

 the influence of a stimulus applied to the nerve, demon- 

 strates the nervous irritability, and may be used as a 

 convenient measure of its intensity. 



The irritability of a motor nerve continues after 

 death. This follows from the foregoing experiment. 

 The irritability of the nerve, like that of the muscles, 

 depends upon its anatomical structure and constitution ; 

 and so long as these continue, the nerve retains its 

 physiological properties. For the same reason, nervous- 

 irritability lasts longer after death in the cold-blooded 

 than in the warm-blooded animals. Various artificial irri- 

 tants may be employed to call it into activity. Pinching 

 or pricking the exposed nerve with steel instruments, the 

 application of caustic liquids, and the galvanic current, 

 all have this effect. Galvanism, however, is the best 

 means for this purpose, as it is more delicate in its opera- 

 ith tion than the others, and will succeed for a longer time. 



Nervous irritability, like that of the muscles, is ex- 

 the pdes'ofa ga hausted by repeated excitement. If an amputated frog's 

 battery (a, leg, with the sciatic nerve attached, be kept in a cool 

 )ie e place, protected from desiccation, the nerve will remain 

 irritable for many hours ; but if excited by repeated stim- 

 ulus, it soon begins to react with diminished energy, and at last ceases 

 to exhibit any further irritability. If now allowed to remain at rest, its 

 irritability will partially return ; and muscular contraction will again 

 ensue on the application of a stimulus to the nerve. Exhausted a second 

 time, and a second time allowed to repose, the nerve will again recover 

 itself ; and this may be repeated several times in succession. At each 

 repetition, however, the recovery of nervous irritability is less complete, 

 until finally it can no longer be recalled. 



Various circumstances tend to diminish or suspend the irritability 

 of motor nerve fibres. As in the case of the sensitive fibres, compres- 

 sion, cold, or other similar agencies will depress the power of the 

 muscular nerves, so that they can no longer excite contraction when 

 subjected to the galvanic current. Severe and sudden mechanical 

 injuries often have the same effect; as where general relaxation, or 

 diminished power of voluntary motion, is produced, in man, by exten- 

 sive contusion or laceration of the limbs. Such an injury produces 

 a disturbance or shock, which affects the entire nervous system, 

 and suspends its irritability; diminishing for the time both mus- 

 cular power and sensibility. It is only after nervous irritability 



FROG'S LEG, 

 the sciatic nerve 

 (N) attached, and 



