THE CIRCULATION. 277 



accelerators for a long period, that the perfused liquid showed no 

 variation in its content of potassium or calcium. 



Langendorff and Engelmann believe that some specific substance is 

 formed in metabolism of the cardiac muscles, a so-called specific inner 

 stimulus is the origin of the heart-beat. The inorganic salts are only 

 accessory in the production of this stimulus. 



Effect of Temperature. The heart of the frog or the excised 

 heart of the mammal is very sensitive to temperature changes. Heat 

 increases the beat and cold slows it, and the variations in the heart-beat 

 vary directly with the changes of temperature. The heart-beat of a 

 cat at 104 degrees F. is four times more rapid than at 59 degrees F. 

 As the temperature of the heart falls the height of the contraction 

 increases up to a certain point, and at the same time the shortening 

 becomes slower and more prolonged. The rate and force of the 

 heart in cold-blooded animals reaches its optimum at about 86 degrees 

 F. Above this the beats decrease in force and also in rate, becoming 



Fig. 89. Variations in Tonus of Auricle of Terrapin's Heart. (FANO. 



irregular. Newell Martin determined the effect of heat on the heart 

 of warm-blooded animals. He found that the highest temperature at 

 which the heart will beat is about 112 degrees F. It ceases to beat at 

 a temperature of 64 degrees F. In fever we have a rapid heart-beat, 

 due to the elevation of temperature. 



Tonus of the Heart. The volume of the heart and the capacity of 

 its cavities are kept always somewhat reduced as a result of a slight 

 tonus in its fibers. 



Heart-muscle,, like other muscle, is never entirely relaxed or com- 

 pletely contracted. Fano has shown that the auricles of the turtle 

 undergo a series of changes in tonus in the muscles, which represent 

 the ordinary contractions of the heart. The venous end of the heart 

 has the greatest tonus. These changes in tonicity are nearly always a 

 result of pressure upon the auriculo-ventricular 'groove. The ventricle 

 rery seldom has alterations of tonus. Fano believes these changes in 

 tonus are myogenic. He thinks these alterations in tonus are due to a 

 substance different from that which causes the usual contractions of 

 the heart. 



