. THE CIRCULATION OF THE BLOOD 299 



material. With its consumption the irritability declines and after a variable 

 period of time the contraction ceases. As the metabolism is far more rapid 

 in the warm-blooded than in the cold-blooded animal, it is probable that the 

 reserved nutritive material is utilized more quickly in the former than in the 

 latter other conditions being equal. So long as it lasts in either class, the 

 irritability and contractility persist. 



Whatever the immediate or exciting cause of the heart contraction may be, 

 the fundamental condition for its manifestation is the maintenance of the 

 irritability. So long as this persists at a sufficiently high level the heart- 

 muscle will contract in response to the appropriate stimulus. 



THE PHYSIOLOGIC PROPERTIES OF THE HEART-MUSCLE 



The physiologic properties of the heart-muscle on which its efficiency as 

 a pumping organ depends, viz., irritability, conductivity, rhythmkity; 

 tonicity, automaticity, have been largely determined by a study of the heart 

 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 has been placed in the appendix, 

 i . Irritability. Irritability of muscle-tissue has been denned as a property, 

 by virtue of which it responds by a change of form to the action of 

 some agent and termed therefore a stimulus. As the heart muscle 

 continues to exhibit changes of form for a variable period of time even 

 after removal from the body it must be admitted that it possesses 

 irritability and that the change of shape is its response 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 



