2 9 o TEXT-BOOK OF PHYSIOLOGY. 



of the frog. As some of the facts to be stated in subsequent paragraphs have 



reference to this heart, it will be found conducive to clearness if its anatomic 



structure and physiologic action be understood. For this reason a brief 



account of the frog heart will be found in the appendix. 



i. Irritability. The heart-muscle in common with other muscles possesses 



irritability, by virtue of which it responds by a change of form to the 



action of a stimulus. Whatever the stimulus, here, as elsewhere, 



there is a conversion of potential into kinetic energy heat, electricity, 



and mechanic motion. The normal physiologic stimulus has not been 



positively determined. In common with other forms of muscle-tissue, 



the heart-muscle may be made to contract by artificial stimuli e.g., 



mechanic, thermic, chemic, and electric. 



For the demonstration of this fact it is necessary to eliminate the 

 action of the physiologic stimulus and to bring the heart to rest in the 

 condition of diastole. This can be done with the frog's heart, by 

 ligating the tissues at the sino-auricular junction, a procedure which 

 prevents the passage of the contraction wave which originates in the 

 sinus, over the auricles and ventricles (a fact that will be more fully 

 alluded to in a subsequent paragraph). With the heart thus prepared 

 and while still in situ, the apex may be connected with a recording 

 lever and its evoked contractions registered on a recording surface. In 

 this condition it will respond by a contraction to any form of an ade- 

 quate stimulus, such as the induced electric current. 



In its irritability, contractility, and manner of response to stimuli, 

 the heart of the mammal corresponds in all essential respects to the 

 heart of the frog or turtle. 



The irritability of the heart-muscle depends primarily on the blood- 

 supply and secondarily on the maintenance of a normal temperature, 

 and so long as both conditions are maintained the muscle will respond 

 by a contraction to any adequate stimulus, physiologic or artificial. 



a. The Blood-supply. The supply of blood to the mammalian heart is 

 derived from the coronary arteries which, though filled during the 

 systole, deliver the blood to the intra-mural arterioles and capillaries 

 during the diastole. The facts relating to the blood-supply have been 

 presented fully in a foregoing paragraph (page 286). 



b. The Influence oj Temperature. For the manifestation of the irrita- 

 bility and contractility it is essential that the heart-muscle be kept at asuf- 

 ficiently high temperature in order that the physiologic or a given arti- 

 ficial stimulus may evoke a maximal contraction. This is accomplished 

 by immersing the suspended heart in a bath of Ringer's solution the tem- 

 perature of which can be readily decreased or increased by appropriate 

 means. The optimum temperature for the frog heart is about 25 C. 

 As the temperature is lowered both rate and force decrease until at 

 about from 4 C. to o C. both cease. Beyond 35 C. it also ceases to 

 contract, because of a coagulation of the muscle substance. The 

 mammalian heart attains its maximum activity at a temperature of 

 37 C. It ceases to beat at about 47 C. on the one hand and at about 

 17 C. on the other hand. 



