THE CIRCULATION OF THE BLOOD 299 



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 294). 



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

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

 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 25C. 

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

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

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

 mammalian heart attains its maximum activity at a temperature of 

 39C. to 4iC. .It ceases to beat at about 47C. on the one hand and 

 at about i7C. on the other hand. 



Conductivity. Conductivity of living material may be defined as the 

 ability to transmit through itself a condition of activity due to the 

 action of a stimulus. In muscle material the condition of activity is 

 characterized by a molecular process known as the excitation process, 

 followed almost immediately by a change of shape known as the con- 

 traction wave. 



In skeletal muscle conductivity is developed to a high degree. Thus 

 if a stimulus, e.g., an induced electric current, be sent transversely 

 through one end of a muscle an excitation process is developed, followed 

 by a contraction wave, both of which are conducted through the muscle 

 without interruption to the other end with a speed, in the frog muscle, 

 of about 10 meters per second. In the cardiac muscle the physiologic 

 stimulus acts at or near the terminations of the venae cavae, from which 

 point an excitation process and a subsequent contraction wave are 

 conducted over the auricles, thence to the ventricles from base to apex 

 with extreme rapidity. It is evident therefore that the heart-muscle 

 also possesses conductivity to a high degree. It is now generally believed 

 that the propagation of both processes is accomplished by muscle-tissue 

 alone, independently of the nerve system. The conductivity, however, 

 is not equally well developed in every part of the heart. 



In the frog heart this is especially true of the tissue at both the sino- 

 auricular and the auriculo-ventricular junctions. At these points the 

 contraction wave is delayed for an appreciable period (a condition at- 

 tributed to the embryonic character of the muscle-tissue), so that what 

 would otherwise be a single wave becomes divided into three smaller 

 waves, so that it becomes possible to observe and distinguish the con- 

 traction of the different chambers of the heart. In the frog s heart 

 the excitation process and the contraction wave begin in the sinus veno- 

 sus, from which they are conducted to the auricles, thence to the ven- 

 tricles. The successive contractions of the walls of the subdivisions o 

 the heart can be readily recorded with suitable apparatus. In Fig. 135, 



