82 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



record with reference to the contraction. Is the contraction of the auricle 

 shown in the tracing? See Rogers, p. 187, for normal heart record. 



Stimulate the ventricle with minimal make or break shocks, but do not 

 let the two stimuli fall within a single cardiac cycle. Try to stimulate 

 at various stages in systole and diastole, but let normal beats intervene 

 between those which are disturbed. By means of simultaneous ordinates 

 determine the moment when the stimulus was applied. Marey discovered 

 that the vertebrate heart is refractory during systole. Cf. Bayliss, p. 454. 

 Why has the heart a long refractory period? According to Ritchie 

 (Nature, 129: 165, 1932) the long refractory period of the heart permits it 

 to recharge beyond the point of stability, hence the spontaneous excitation. 



6. The Recovery of Excitability Following a Response. — In Ex. 3 

 you were cautioned to wait 15 seconds between successive excitations. 

 The following experiment will show why. 



Use the preparation described in Ex. 3, (with ligature). Insert a 

 signal magnet in the primary circuit and arrange the inductorium so that 

 the make shock is just adequate to excite. With the drum rotating very 

 slowly stimulate with pairs of inductorium induction shocks, the make 

 following the break at gradually decreasing intervals (3 seconds and less) 

 until no response is obtained. The interval between the pairs of stimula- 

 tions should be constant and at least 15 seconds in length. The second 

 curve will drop out when the refractory interval is reached. 



7. Measurement of Changes in Irritability (A tedious experiment). — 

 Use a fresh preparation with a Stannius ligature. Start with the induc- 

 torium in a position such that the make shock is just effective when applied 

 to the fully recovered muscle. Stimulate with a break shock followed by 

 a make shock after an interval of 7 sees. Keeping the interval at 7 sees, 

 increase the distance between the primary and secondary coil one mm. 

 at a time until the make shock becomes ineffective. A rest interval of at 

 least 15 sees, should separate the pairs of excitations. Record the posi- 

 tion at which the make shock is least effective with a 7 sec. interval. 



Repeat, reducing the interval between break and make to 6, 5, 4, 3, 2, 

 and 1 sees., and fractionating the intervals further if you can do so. The 

 intervals may be represented: break 7 sees., make — 15 sees, wait — break 

 6 sees, make — 15 sees, wait — break 4 sees, make — 15 sees, wait, etc. At 

 end of 7 sec. interval move coil to find threshold DISTANCE. 



The heart becomes refractory for a certain time interval after the first 

 shock. The position of the secondary coil for the second shock (make) 

 measures the excitability at the particular time interval, i.e., 7, 6, 5, 4, 3, 

 2, 1 sees, after the initial beat. Immediately following the first stimula- 

 tion no response is obtained on the second shock. At longer time inter- 

 vals you are testing the recovery of the heart. Plot the positions of the 

 secondary coil, which may be taken as a measure of the excitability of the 



