THE HEART 59 



the functions of the cervical sympathetic by perfusing with Ringer's 

 solution, whereas the irritability of the vagus could be restored for 

 some time, and that of the acceletor almost indefinitely. 



It is in the tissue of the sinus in the frog's heart that the 

 automatic power resides. This is the part of the heart which can 

 be affected by such agents as heat and cold. Destruction of the 

 sinus or cutting it off alters the rhythm of the remaining portions 

 of the heart. One would expect, if the results obtained upon the 

 frog's heart be applicable to the mammalian heart, that similar 

 results should be obtained by experimenting upon the parts repre- 

 senting the sinus. Such indeed is the case. At the junction of 

 the superior vena cava with the auricle Adam has succeeded in 

 altering the mammalian heart rhythm by the application of heat 

 and cold. Hering has shown that the rhythm in the dog can be 

 completely changed by a cut in this region. Langendorff has 

 obtained somewhat similar results with the Stannius ligature 

 experiment upon the mammal. The results were not so marked 

 as with the frog, the period of rest being slight, and the heart 

 resuming its beat more quickly, although with a different rhythm. 

 As we have seen, the sino-auricular node is situated in this region, 

 and it may well be that this is the spot acted upon in these experi- 

 ments. Experimental evidence to this effect, however, is still 

 wanting, and the true function of this node has yet to be worked 

 out. In the meantime it has been suggested that in view of 

 its embryonic structure it is likely to possess great automatic 

 properties, and that seeing how carefully it is supplied with blood, 

 and how intimately the nerves of the heart come into contact 

 with it, this isolated node of tissue is probably the site of origin 

 of the heart's impulse. On the other hand, also without physio- 

 logical evidence, Tawara has claimed that the heart's impulse 

 normally arises in the A-V node, which he has termed the " cardio- 

 motor centre." No proof of this is as yet forthcoming, but the 

 view more generally held, although as yet without definite evi- 

 dence, is that the A-V node must be regarded as a subsidiary centre, 

 and that it is only under certain circumstances that the heart 

 rhythm is initiated there. While dealing with such hypothetical 

 problems it may be suggested that the possible explanation of the 

 effect of the first Stannius ligature is due to the cutting off of the 

 automatic tissue at the sinua, i.e. in the case of mammalian heart 

 possibly the sino-auricular node. Under these circumstances the 



