508 CIRCULATION OF BLOOD AND LYMPH. 



ventricle, for instance is due, in the heart of the tortoise, to the 

 fact that the muscular tissue at the junction of auricle and ventricle 

 has a relatively low rate of conduction. At this point, indeed, 

 the muscular fibers form a ring around the orifice, preserving, 

 therefore, the arrangement found in the embryo at the time that 

 the heart has the form of a tube. Gaskell has given reasons for 

 believing that the conduction of the wave of contraction is slower 

 through this ring. In the mammalian heart the direct conduction 

 of the wave of contraction from auricle to ventricle through interven- 

 ing muscular tissue is made quite possible, since so many indepen- 

 dent observers have established the existence of a connecting 

 bundle (p. 479). If with Gaskell we assume that the conduction 

 through this bundle is slower than it is over the surface of the auricle 

 or ventricle, then the pause between auricular and ventricular 

 systole is sufficiently explained. That each chamber of the heart has 

 a rhythm of its own and that the rhythm of the venous end is the 

 more rapid and constitutes the rhythm of the intact heart has been 

 shown in various ways upon the hearts of different animals. Thus, 

 Tigerstedt has devised an instrument, the atriotome,* by means of 

 which the connections between auricle and ventricle may be 

 crushed without hemorrhage. Under such conditions the ventricle 

 continues to beat, but with a much slower rhythm and with a 

 rhythm entirely independent of that of the auricles. The same re- 

 sult has been obtained recently in a very striking way by Erlanger. 

 This observer arranged a clamp by means of which he could com- 

 press the small bundle of fibers connecting auricle and ventricle. 

 When the compression is made the ventricle, after an interval, 

 exhibits a slower rhythm and one entirely independent of that of 

 the auricles. When the compression is removed the ventricle falls 

 in again with the auricular rhythm. By variations in the pressure 

 upon the bundle intermediate conditions may be obtained in which 

 the "block" between auricle and ventricle is only partial and in 

 which, therefore, the ventricular systole follows regularly every 

 second or third auricular contraction. When the "block" is com- 

 plete the ventricular rhythm ceases to have any definite relation- 

 ship to that of the auricle, it beats entirely independently and its 

 rate is slower than that of the auricle. It is interesting to remember 

 that cases of complete or partial heart block occur in man. In 

 the condition known as the Stokes- Adams syndrome the striking 

 feature in addition to attacks of syncope is a permanently slowed 

 pulse, the heart beat falling to 30 or 20 beats per minute or lower. 

 Erlanger has shown that in such cases there may be complete or 

 partial heart block. In the former condition the rhythm of the 

 ventricle is entirely independent of that of the auricle and of course 

 * See " Lehrbuch der Physiologie des Kreislaufes," 1893. 



