CHAPTER XII. 



EFFECT OF SNAKE VENOM UPON THE NERVOUS SYSTEM 



AND EFFECT OF THE SEQUEL/E UPON THE RESPIRATORY 



AND CIRCULATORY FUNCTIONS. 



CROTALIN^I. 



S. Weir Mitchell demonstrated as early as 1861 that the toxication of frogs 

 and rabbits by the bite of the rattlesnake quickly stops all motion, volun- 

 tary or reflex, within a short time after the bite. In frogs it was found that 

 no reflex acts respond to the stimulation of the sciatic nerves, although the 

 galvanization is followed by the movements of the muscles of the same leg. 

 On the other hand, the electric stimuli to the sciatic nerves, from which the 

 vicious effect of the poison had been excluded by cutting off all of its cir- 

 culation, produced contraction of the muscles. The loss of the motion was 

 then thought to be caused either by the incapability of the sensory nerves 

 to transmit the stimulus to the nervous centers or by the destruction of the 

 nerve centers themselves. In order to ascertain this point he poisoned a 

 frog to a state of total paralysis, then cut the spinal cord across and thrust a 

 probe up and down. No motion resulted. The irritability of the sciatic 

 nerves when directly tested was found to be nearly perfect. Thus the dis- 

 appearance of motion seems to have been caused by the paralysis of the 

 spinal cord. 



In rabbits it was necessary to keep up the circulation and cardiac activity 

 after the cessation of respiration by means of artificial respiration, which 

 seems to sustain the heart's action for about 12 minutes. During this stage 

 the spinal cord was cut across; no motion resulted. A probe being thrust up 

 and down the spine, feeble quivering of the nearer spinal muscles took place, 

 but the limbs did not move. On dividing the sciatic nerves free motion was 

 observed, and the phrenic trunk was likewise excitable. 



In 1886 Weir Mitchell and Reichert made further detailed observations 

 on the effects of Crotalus adamanteus, Ancistrodon piscivorus, and Naja 

 tripudians. Their experiments chiefly concern the effects of the crotaline 

 venoms. In administering a dose of these venoms — just enough to cause 

 acute or subacute poisoning in rabbits — into the jugular vein, they always 

 noticed the increase in the respiration rate, which, however, was quickly 

 followed by a diminution far below the normal. Corneal reflex disappeared 

 before the cessation of respiration, which occurred first. In testing the 

 electric excitability of the respiratory muscles they found them responsive 

 to the stimulus. After exposing the spinal cord the sensory column was 



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