ACTION OF THE HEART. 311 



fibres of the Vagus that the heart's action may be retarded than is required 

 to .stimulate the recurrent laryugeal fibres, so that the laryngeal muscles 

 may be thrown into action. The accompanying woodcut will show by means 

 of a tracing the effect of stimulation of the lower end of the Vagus in the 

 rabbit. It will be observed that the undulations, each of which represents 



FIG. 128. 



After. During. Before. 



Tracing showing effect of stimulation of Vagus. The tracing must be read from right to left. 



a cardiac contraction, are very rapid and small till the moment at which 

 the stimulus was applied to the Vagus. They then become much slower, 

 and larger, whilst their mean level falls, indicating a diminution of blood- 

 pressure. As soon as the stimulus to the Vagus is withdrawn the contrac- 

 tions present their former frequency, and the blood-pressure again rises. 

 Further inquiry by Cyon and Ludwig 1 soon showed that in the rabbit the 

 Vagus gives off a branch immediately below the inferior laryngeal nerve, 

 which may be called the Cardie-inhibitory of the Vagus (see Fig. 127), 

 since when excited it is capable of exerting the same controlling, restrain- 

 ing, arresting, or inhibitory power over the Cardiac movements as the Vagus 

 itself. In man this nerve is represented by the superior Cardiac branch of 

 the Vagus. If this nerve be divided and its upper cut surface be irritated 

 no effects are produced, but if the lower cut surface be irritated, the heart 

 stops in diastole. The fibres of the nerve therefore appear to conduct im- 

 pulses downwards (as indicated by the arrow in the figure). It might be 

 supposed that these fibres pass to the Cardio-motor centre, and exert a direct 

 inhibitory influence upon it, but several circumstances render it probable 

 that an intermediate apparatus or inhibitory mechanism which we may 

 term an inhibitory centre exists in the heart between the vagal branches 

 running to the heart and the ultimate fibrils of their branches. For, in the 

 first place, when the Cardio-iuhibitory nerves are irritated it has been shown 

 both by Pfliiger and Bonders that a very appreciable interval elapses before 

 the inhibitory action is manifested. Thus Pfliiger found that even when the 

 stimulus applied was strong enough to produce complete arrest of the Car- 

 diac movements, two entire beats always intervened before the cessation 

 occurred, 2 and in Douders's experiments, a period of 0.17 sees, elapsed before 



1 Ludwig's Arbeiten, 1867, p. 128. 



2 Pfliiger found that the first perceptible effect of slight irritation of the Vagi is a 

 prolongation of the diastole of the heart, or a retardation of the motor impulse, in- 

 ducing the next contraction. When violent or long-continued excitation is applied 

 to the Vagi, producing stoppage of the heart's action, the first beats on its recovery 

 are usually very feeble; but when only moderately strong currents are applied, the 

 first beats are often as energetic or even more powerful than before. This would 



