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TEXT-BOOK OF PHYSIOLOGY 



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 con- 

 tinued 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 ex- 

 tent as to prevent absolutely the passage of 

 the contraction wave by ligation of the tis- 

 sue between them, a procedure introduced 

 by Stannius and now known as the first 

 Stannius ligature. /Under such circum- 

 stances the auricles and ventricle remain at 

 rest while the sinus continues to beat at its 

 usual rate. The obstacle between the auri- 

 cles and ventricle may be increased by the 

 same method also introduced by Stannius, 

 and now known as the second Stannius 

 ligature, or better by means of a suitable 

 and adjustable clamp. By carefully regulating the pressure of the clamp 

 it is possible to so block the wave that three or four auricular contrac- 

 tions may occur before the excitation process forces the block and ex- 

 cites a ventricular contraction. (Fig. 136.) If the block is complete, 

 rather than partial, .the ventricle will come to rest and so remain. 

 From the foregoing facts it is evident that the physiologic stimulus ex- 

 erts its action in the sinus venosus and that the auricular and ven- 

 tricular beats are in turn dependent on it. 



I/ I/ c l/ V 



FIG. 135. RECORD or THE 

 CONTRACTION OF THE FROG'S 

 HEART. 



FIG. 136. RECORD or THE AURICULAR AND VENTRICULAR CONTRACTIONS BEFORE 



AND AFTER THE CLOSURE OF THE CLAMP AT a. 



In the mammalian heart the seat of the stimulus and the point oi 

 origin of the excitation process and the subsequent contraction wave 

 have been a subject of much investigation and discussion. For some 

 time it has been believed that these processes originate at the termina- 

 tions of, or between the terminations of the venae cavae in a region 

 corresponding to the sinus venosus in the frog heart 1 , from which they 



J In the mammalian heart the sinus venosus as a distinct chamber has been obliterated, 

 but it is represented by the following remnants: (i) The termination of the superior vena 



