78 ELEMENTARY PHYSIOLOGY. XIV. 



stimulus. Repeat this several times; the contraction 

 may increase gradually in height (staircase effect) but 

 this depends upon the condition of the heart. 



Stimulate with the break current immediately follow- 

 ing the make ; a single contraction only will be obtained 

 of the same height as before, i.e. for a short time after 

 beginning to contract the ventricle is not irritable, it 

 has a rather long refractory period. 



Stimulate again, gradually increasing the interval be- 

 tween the break and the make current ; when the break 

 stimulus falls in the relaxation period, a contraction will 

 be obtained, the extent of which increases the greater 

 the extent of relaxation, but the actual freight does not 

 exceed that of the contraction at make. 



Decrease the strength of the currents, as long as a 

 contraction is obtained, its height does not vary (law 

 of maximal response). 



Stimulate with tetanising currents; a series of beats 

 are caused and not a continuous contraction. 



b. If the heart is beating: 



Stimulate with break currents at various periods of 

 the spontaneous contraction, and with tetanising cur- 

 rents, the latter will be followed by a long period of 

 quiescence. 



