254 PHYSIOLOGY OF THE HEART [CH. XX. 



Blocking. This phenomenon has been chiefly studied by Gaskell. 

 It appears that under normal conditions the wave of contraction in 

 the heart starts at the sinus, and travels over the auricles to the 

 ventricle ; the irritability of the muscle and the power of rhythmic 

 contractility is greatest in the sinus, less in the auricles, and still less 

 in the ventricles. Under ordinary conditions the apical portion of 

 the ventricles exhibits very slight power of spontaneous contraction. 

 The importance of the sinus as the starting-point of the peristalsis 

 can be shown by warming it. If a frog's heart is warmed by bathing 

 it in warm salt solution at about body temperature, it beats faster ; 

 this is due to the sinus starting a larger number of peristaltic waves ; 

 that this is the case may be demonstrated by warming localised portions 

 of the heart by a small galvano-cautery ; if the sinus is warmed the 

 heart beats faster, but if the auricles or ventricles are warmed there 

 is no alteration in the heart's rate. The sinus in the frog's heart, 

 and that portion of the right auricle in the mammal's heart which 

 corresponds to the sinus, is always the last portion of the heart to 

 cease beating on death, or after removal from the body (ultima 

 moriens, Harvey). This is an additional proof of the superior rhyth- 

 mical power which it possesses. 



But to continue our description of the phenomenon known as 

 blocking ; it is supposed that the wave starting at the sinus is more 

 or less blocked by a ring of lower irritability at its junction with the 

 auricle; again, the wave in the auricle is similarly delayed in its 

 passage over to the ventricle by a ring of lesser irritability, and thus the 

 wave of contraction is delayed at its entrance into both auricular and 

 ventricular tissue. By an arrangement of ligatures, or, better, of 

 clamps, one part of the heart may be isolated from the other portions, 

 and the contraction when aroused by an induction shock may be 

 made to stop in the portion of the heart muscle in which it begins. 

 It is not unlikely that the contraction of one portion of the heart acts 

 as a stimulus to the next portion, and that clamps and ligatures prevent 

 this normal propagation of stimuli. It must not, however, be thought 

 that the wave of contraction is incapable of passing over the heart in 

 any other direction than from the sinus onwards; for it has been 

 shown that by the application of appropriate stimuli at appropriate 

 instants, the natural sequence of beats may be reversed, and the con- 

 traction starting at the arterial part of the ventricle may pass to the 

 auricles and then to the sinus. 



If Gaskell's clamps or ligatures are not applied sufficiently tight 

 one often sees partial blocking, a few waves get through but not all ; 

 in the experiment described on the preceding page, in which the 

 heart wall is left connected with other parts by a small portion of 

 undivided muscular tissue, the effect is much the same, the wave is 

 only able to pass the block every second or third beat. 



