THE CIRCULATION OF THE BLOOD. 291 



2. 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 de- 

 layed for an appreciable period (a condi- 

 tion attributed to the embryonic charac- 

 ter 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 dis- 

 tinguish the contraction of the different 

 chambers of the heart. In the frog s HEART. 

 heart the excitation process and the con- 

 traction wave begin in the sinus venosus, from which they are 

 conducted to the auricles, thence to the ventricles. The successive 

 contractions of the walls of the subdivisions of the heart can be 

 readily recorded with suitable apparatus. In Fig. 135, which is 

 a graphic record of the heart-beat, the two elevations of the lever on the 

 up-stroke, a and b, represent the contraction of the sinus and the auricle 

 respectively, followed by the vigorous and long continued contraction 

 of the ventricle, while the two depressions, c and d, indicate the delay 

 in the transmission of the contraction wave at the two junctions. There 

 is here an anatomic obstacle to the conduction of the contraction wave. 

 The block between the sinus and the auricle may be artificially 

 increased to such an extent as to prevent absolutely the passage of the 

 contraction wave by ligation of the tissue, as first suggested by Stannius. 

 Under such circumstances the auricles and ventricle remain at rest 

 while the sinus continues to beat at its usual rate. The obstacle between 

 the auricles and ventricle may be increased by the same method or 



