296 TEXT-BOOK OF PHYSIOLOGY. 



Aur. rate per minute. Ven. rate per minute. Ratio of aur. to Ven. 



79-6 22.4 3.55 



84.6 31.0 2.73 



In a few cases of death from this disease a post-mortem examination 

 showed a lesion of the auriculo-ventricular bundle. 



3. Rhythmicity. Rhythmicity may be denned as the ability to act in 



regularly recurring cycles or the property of anything so acting. As 

 the heart-beat recurs in regular cycles or at regular intervals, it may 

 therefore be said that the heart-muscle is characterized by rhythmicity. 

 The beat of the heart as well as each phase of the beat occupies a 

 regular measure of time and is therefore rhythmic in character. Experi- 

 mental procedures, however, show that the rhythmic power or at least 

 the frequency of the rhythm varies in each of its subdivisions when they 

 are separated one from the other. Thus if the tissue between the sinus 

 and auricle in the frog or turtle heart be divided, the auriculo-ventricular 

 portion at once ceases to beat, while the sinus continues to beat as usual. 

 In a short time, however, the auricles and ventricles begin again to beat, 

 but with a slower rhythm. Division of the tissue between auricles and 

 ventricles is again followed by rest. In a short time the auricles begin 

 to beat, while the ventricle remains quiescent. If the ventricle now be 

 stimulated in a rhythmic manner it may resume rhythmic activity. 

 These facts are taken as an indication that the rhythmic power is 

 greatest in the sinus, less in the auricles, and least in the ventricles. 

 In the warm-blooded animal, e.g., dog, cat, rabbit, there is also a 

 difference in the rhythmicity of the auricles and ventricles. This is 

 shown by the effects which follow division of the auriculo-ventricular 

 bundle, or sudden and complete compression of that portion of the 

 auriculo-ventricular tissue containing it. In either case the ventricle 

 for a short time remains at rest, though the auricles continue to beat at 

 their usual rate. After a variable number of seconds the ventricle 

 develops a rhythm of its own, though it never attains that of the auricle. 

 From these facts it is probable that in each division of the heart a stimu- 

 lus similar to that acting in the sinus is developed when the heart 

 chambers are separated one from the other. 



4. Tonicity. Tonicity may be denned as a condition of muscle material 



characterized by a slight degree of contraction which varies in extent, 

 however, from time to time under physiologic conditions. Whatever 

 the cause of the tonicity may be in any given form of muscle, the 

 slight degree of contraction which characterizes it not only resists undue 

 extension but permits of a quicker response to the action of a stimulus 

 and a more effective performance of work. The heart-muscle, like 

 the skeletal muscle, maintains continuously a certain degree of contrac- 

 tion, which not only prevents undue expansion of the heart during the 

 period of diastole, but increases its efficiency as a pumping organ at the 

 beginning and during the systole. This tone may, however, be increased 

 or decreased by the action of various external agents. Thus the passage 

 of dilute solutions of various drugs e.g., alkalies, digitalis through 

 the cavities of the excised heart will so increase the tonicity, or the 



