THE HEART 51 



exhibit this phenomenon. The refractory period is in all proba- 

 bility connected with the internal metabolic processes concerned 

 in the building up of the heart's contractile material, and it 

 appears at present impossible to separate it from the fundamental 

 processes of rhythmicity. 



The direction of a contraction through the mammalian heart 

 is from auricles to ventricles via the auriculo- ventricular bundle ; 

 the rate of conduction through this bundle is slower than that 

 through the ordinary heart muscle, which explains the slight 

 pause between the contraction of the auricles and that of the 

 ventricles. The rhythm set by the auricles is under normal 

 conditions taken up by the ventricles ; but experimentally it 

 can be shown that interference with the A.V. bundle may lead 

 to the rhythm of the ventricles being slower and independent of 

 that of the auricles. The precise results obtained depend upon 

 whether there is a partial or complete block between the auricles 

 and ventricles. The condition thus experimentally produced has 

 its clinical counterpart in irregularities due to heart-block. 



One word more may be said in connection with the transmis- 

 sion of a contraction through the heart. Chauveau has shown 

 that the two auricles in the horse do not contract precisely 

 together ; there is a slight delay in the contraction of the left, 

 and this is explained by saying that the transmission of the 

 contraction from right auricle to left auricle takes time. 



Nervous Mechanism of the Heart. — Up to this point the ques- 

 tion of the rhythmical contraction of the heart has been con- 

 sidered ; we have now to take up another and distinct question — 

 viz., the influence of the nervous system in regulating the activity 

 of the heart. The heart receives a nerve supply from two 

 portions of the central nervous system : one is concerned with 

 the transmission of impulses which slow or stop the heart, hence 

 called inhibitory ; the other conveys to the heart impulses which 

 stimulate or augment the activity, and are in consequence 

 known as augmentor or accelerator nerves. From the vagus 

 is derived the inhibitory effects, and from the sympathetic the 

 augmentor. It is obvious that these nerves are antagonistic, 

 the one endeavouring to slow the heart, the other pressing it on. 

 The balance between these two opposite conditions results in 

 the normal rate of heart-beat. 



Not only have these two nerves opposite functions, but they 

 are also structurally different, the vagus being a medullated, the 

 sympathetic a non-medullated nerve. 



An immense amount of work has been done in endeavouring 

 to elucidate the physiological effect of the vagus and sympathetic 

 on the heart, and the results have not always been concordant. 

 This may be due to the fact that the frog has furnished the bulk 



