THE PHYSIOLOGY OF THE HEART 141 



the normally beating heart (Figs. 48, 49). In the dog it causes a 

 slight slowing, in the rabbit it has no effect. Under these circum- 

 stances, the electro-cardiogram shows that the excitatory wave now 

 arises in the A.-V. node. 



But excision of the A.-V. node in the well-nourished normal heart, 

 beating in situ, does not stop either auricles or ventricles. The 

 auricles beat as before, and the ventricles with a slow independent 

 rhythm of their own. 



The nodal tissue, therefore, cannot be regarded as the sole re- 

 pository of the automaticity of the heart. It is apparently longer 

 lived than the other parts; so that under conditions of malnutrition 

 the excision of the tissue may cause cessation of the heart-beat. It 

 is in the areas of the nodes that the dying heart beats last, and beats 

 first when restored by perfusion. 



Further, the bird's heart, which possesses a very high degree of 

 automaticity, possesses no nodal tissue. 



In the bird's heart, heartblock may be induced by the ligature of 

 the muscular connection between auricles and ventricles (Fig. 50). 

 This connection does not run in the position of the A.-V. bundle, but 

 posteriorly in the outer wall of the right side of the heart. It is 

 possible that there is also a connection between auricles and ventricles 

 in the mammalian heart in this region, but if there be, it is not the 

 path of the normal excitatory wave of the heart. 



To sum up, it appears that the evidence at present available 

 supports the view that the excitatory wave of the mammalian heart 

 arises normally in the sinu-auricular node, and spreads over the 

 auricular muscle, and thence to the ventricles by the musculature of 

 the A.-V. bundle. The retardation which takes place in the A.-V. 

 bundle causes the ventricle to beat at the proper period after the 

 auricle. The A.-V. bundle normally conducts the excitatory wave 

 to the different parts of the ventricles, so that these parts contract 

 co-ordinately, and wring the blood out of the heart. 



Although the property of rhythmic automaticity is highly developed 

 in the nodal tissue, this tissue is not to be regarded as the only tissue 

 of the heart possessing automaticity. The ordinary musculature of 

 the auricles is also endowed with this property, and that of the ven- 

 tricles to a less degree. 



