74 ELEMENTARY PHYSIOLOGY less. 



If a frog be pithed, or its brain be otherwise destroyed, 

 so as to obliterate all sensibility, the animal will continue 

 to live, and its circulation will go on perfectly well for a 

 prolonged period. The body may be laid open without 

 causing pain or other disturbance, and then the heart will 

 be observed beating with great regularity. It is possible to 

 make the heart move a long lever backwards and forwards ; 

 and if frog and lever are covered with a glass shade, the 

 air under which is kept moist, the lever may vibrate with 

 great steadiness for a couple of days. 



It is easy to adjust to the frog thus prepared a contri- 

 vance by which electrical shocks may be sent through the 

 vagus nerves, so as to stimulate them. If the stimulation 

 is only gentle or weak, the heart will be seen to beat 

 more slowly, and at the same time each beat is rather 

 more feeble, as shown by the diminislied distance over 

 which the end of the lever moves. But if the stimulation 

 is strong, the lever almost innnediately stops dead, and 

 the heart will be found quiescent, with relaxed and dis- 

 tended walls. After a little time the influence of the 

 vagus passes off, the heart recommences its work as 

 vigorously as before, and the lever vibrates through the 

 same arc as formerly. With careful management, this 

 experiment may be repeated very many times ; and after 

 every arrest by the stimulation of the vagus, the heart 

 resumes its work. 



If on the other hand the stimulation be applied to the 

 sympathetic nerves, then an eftbct is produced which is 

 exactly the opi)osite to that which results from stimulating 

 the vagus. The lever moves more rapidly and over a 

 greater distance, showing quite clearly that the heart is 

 now beating faster and tliat each beat is stronger. 



No clearer proof could be desired than is afforded by 

 the above experiments, that the heart of the frog is con- 

 trolled by two antagonistic nerves of which one, the 

 vagus, carries impulses which slow and finally stop its 



