_'75- Inhibition />v direct stimulation of 'raijus. Kxpose the 

 vagus nerve in frog as follows: 1'a a rather wide glass tube down 

 the oesophagus i to put the tissues on the stretch ). This will expose 

 three large ner\ e- at the side of the neck; these are. in order from 

 above down, glosso-pharyngeal, -I'lu/ns. liv^oi/lossus. Stimulate the 

 vagus with a weak tctani/.ing current, noting the effect on heart. 

 If no ettect is shown try a stronger current, <>r try tire vagus on the 

 oilier sido ot the body, since the two are often unequal in their 

 action. Xote ( i ) latent ])eriod of inhibition. (2) duration of inhibi- 

 tion. ( ^ i manner in which the beats are resumed. 



Take a record of vagus inhibition, placing a signal magnet in the 

 circuit to record the moment of stimulation. Then connect the signal 

 magnet with the desk binding post and revolving the drum at the 

 same -peed take a second's time curve. 



1 1 the frog's heart is weak tise a turtle. The vagus lies in the 

 side of the neck and may be exposed bv putting the tissues mi the 

 stretch, and recognized by stimulation. 



To make sure that the effect is not reflex, ligature the vagus as 

 near the central end as possible, cut centrally to ligature, lift by 

 ligature and stimulate. Test the direct irritability of the heart while 

 in a state of inhibition. Does it respond readily? 



IV. EFFECT OF VARIOUS FACTORS ON Till-: CHARAC- 

 TER < )!' THE RHYTHM. 



2~(i. (ini[>luc record of the influence of temperature on the rate 

 of heart heat. Kxpose the heart of a frog. Pass a small hook at- 

 tached to a thread through the tip of the ventricle. Then excise the 

 whole heart, cutting widely around it. and pin the tissues surrounding 

 its base to a small cork plate, hasten the plate to a glass rod by in- 

 serting the latter into a hole cut with a cork borer. The heart, thus 

 attached to the rod. may then be immersed in anv desired solution, 

 and its action recorded by the thread, which is tied to the end of the 

 short arm of the heart lever. 



The heart is immersed in Kingcr's solution contained in a glass or 

 beaker supported by a block, as in the experiments with voluntary 

 muscle. Take' tracings of the beat on a slow drum with the heart 

 surrounded bv Kinger's solution at the temperatures 5 . 15 . 25 . 

 Proceed thus; Have ready a glass filled with the cold Kinger, e.g.. 

 5; bring the heart into the solution in the Usual manner ( bv re- 

 moving the block, bringing the solution up from below, and then 

 replacing block). Let the heart make a tracing at tin's tempera- 



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