256 PHYSIOLOGY OF THE HEART. [CH. xx. 



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 the 

 sinus contraction generally begins first, since the sinus is the 

 most irritable to stimuli, and possesses the power of rhythmic 

 contractility to the most highly developed degree. It must not 

 be thought, however, that the wave of contraction is incapable of 

 passing over the heart in any other direction than from the sinus 

 onwards ; it has been shown that by application of appropriate 

 stimuli at appropriate instants, the natural sequence of beats may 

 be reversed, and the contraction starting at the arterial part of 

 the ventricle may pass to the auricles and then to the sinus. 



The question has been discussed whether the wave from one chamber is 

 passed on to the next by nervous or muscular connection. The slow rate 

 of propagation of the wave and the existence of muscular fibres passing 

 across from one chamber to the next show that the link is a muscular one. 



An exceedingly interesting fact with regard to the passage of 

 the wave in any direction has been made out by partial division 

 of the muscular fibres at any point, whereby one part of the wall 

 of the heart is left connected with the other parts by a small 

 portion of undivided muscular tissue, and the wave of contraction 

 is then able to pass to the next portion of the wall only every 

 second or third beat. Thus division of the muscle has much 

 the same effect as partial clamping it in the same position, or of 

 a ligature similarly applied, but not tied tightly. The first 

 Stannius ligature acts as a partial or complete block, and pre- 

 vents the stimulus of the sinus-beat from passing further down 

 the heart, but parts beyond the ligature may be made to contract 

 by stimuli applied to them directly. The second Stannius liga- 

 ture acts as a stimulus to the ventricle. Instead of applying 

 the second ligature, the experiment may be varied by cutting off 

 the heart beyond the first ligature ; the stimulation caused by 

 cutting produces waves that travel over auricles and ventricle. 



The explanation of the action of the Stannius ligatures just given can 

 hardly be regarded as wholly satisfactory though it is the best that can be 

 offered at present. If the first ligature acts as a block it is difficult to under- 

 stand why the second should act as a stimulus ; and if the second acts as a 

 stimulus, the question arises why the first should not act as a stimulus also. 



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

 stalsis can be shown by warming it. If the heart is warmed by 

 bathing it in salt solution at about the body temperature, it beats 

 faster ; this is due to the sinus starting a larger number of peri- 

 staltic waves ; that this is the case may be demonstrated by warm- 

 ing localised portions of the heart by a small galvano-cautery ; if 



