304 TEXT-BOOK OF PHYSIOLOGY 



been termed the 2 .: i, 3 : i, 4 : i, rhythms respectively; and finally 

 when the blocking is complete no excitation process can reach the 

 ventricle. 



Owing to the capability of the mammalian ventricle to develop an 

 independent rhythm when not stimulated by the auricles for a few 

 seconds or more, it is not always possible to state at what particular 

 moment in the successive stages of compression the independent ventricu- 

 lar rhythm becomes manifest. Usually when the rhythm is of the 3 : i 

 type, i.e., when the third auricular contraction fails to reach the ventricle, 

 it will begin to beat of itself. Under such circumstances the auricles 

 and ventricles become dissociated even though the block is not quite 

 complete. 



These experimental facts have afforded an explanation of the altered 

 rhythm between auricles and ventricles often found in that pathologic 

 condition known as Adams-Stokes disease. In this disease the rhythm 

 may be any one of the rhythms stated in the foregoing paragraph. In 

 two instances the following ratio of the ventricle to the auricle was 

 observed by Erlanger. 



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 defined 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 



