184 CIRCULATION OF THE BLOOD 



into an actual tetanus, like skeletal muscles. Since all the stimuli which 

 fall during systole are entirely ineffective, there can be no superposition and 

 summation of effects. 



This rule is not strictly without exception, however. By simultaneous 

 stimulation of the vagus and the venous sinus 0. Frank was able to demon- 

 strate a condition of tetanus in the frog's heart; Walther observed the same 

 thing on stimulation of a frog's heart poisoned with muscarine. In the 

 latter case the refractory period of the heart is shortened as a result of the 

 poison, and the barrier to the production of tetanus is thereby removed. 



Many other discoveries have been made on the exsected heart concerning 



FIG. 66. Influence of temperature on the isolated heart of the cat, after Langendorff. The 

 heart was supplied with blood kept at the different temperatures indicated. 



the properties of heart muscle, but they cannot be discussed here. We wish 

 to call attention only to the following peculiarities: 



(1) The frequency of the heart beat varies directly with the temperature 

 i. e., the higher the temperature the greater the frequency, the lower the 

 temperature the less the frequency. Thus at 40 C. it is four times as great 

 as at 15 C. With a fall in the frequency, the extent of contraction increases 

 up to a certain limit, and at the same time the shortening becomes slower 

 and more drawn out (Fig. 66). 



(2) The quantity of blood which the left ventricle discharges at each 

 systole depends upon the quantity of flow from the great veins : the greater 

 the inflow the greater the amount discharged, but the latter increases more 

 slowly than the former. 



(3) The quantity of blood flowing through the coronary vessels exercises 

 but little influence upon the frequency, although it is of great importance for 

 the force of the heart beat. 



(4) If the heart has no work to do it has need of only a very small quan- 

 tity of blood. 



