292 TEXT-BOOK OF PHYSIOLOGY. 



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 contractions may occur before the excitation 

 process forces the block and excites 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 exerts its action in the sinus venosus and that 

 the auricular and ventricular beats are in turn dependent on it. 



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

 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 



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

 AND AFTER THE CLOSURE OF THE CLAMP AT a. 



pass over the auricles, thence to the ventricles. On the basis of this belief 

 it has been assumed that there is a specialized area in which the stimulus 

 arises and which determines the rate and rhythm of the entire heart. 

 At present it is believed that this area is identical with the region occu- 

 pied by the sino-auricular node, the lower portion of the sulcus terminalis. 

 With the view of determining the truth of this assumption Flack per- 

 formed a number of experiments on the hearts of dogs, cats, and rabbits, 

 some of the results of which, abstracted from his paper, are as follows : 

 The application of cold either through metallic tubes or by means of an 

 ethyl chlorid spray, the remainder of the heart being protected, caused 

 slowing of both auricles and ventricles. Weak electric stimulation 

 caused marked inhibition of both auricles and ventricles; slightly 

 stronger stimulation caused a mixed effect of inhibition and acceleration, 

 the latter usually predominating; still stronger stimulation gave rise to 

 marked acceleration of the whole heart rhythm or an altered rhythm of 



1 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 

 cava (the right duct of Cuvier); (2) the coronary sinus (the left duct of Cuvier); (3), a 

 stratum submerged beneath auricular tissue at the tsenia terminalis; (4) the remnants of 

 the venous valves, i.e., the Thebesian and Eustachian valves (Flack). In addition there 

 is a remnant of primitive tissue at the sino-auricular junction, that is, where the superior 

 vena cava joins the taenia terminalis of the right auricle, and known as the sino-auricular 

 node. 



