163 THE CIRCULATION OF THE BLOOD AND LYMPH 



Whatever the exact mechanism of augmentation may be, there is 

 no basis for the statement that the cardio-augmentor nerves have 

 an action on the heart so fundamentally different from the action of 

 motor nerves on skeletal muscle that they cannot originate contractions 

 in a heart entirely at rest. Excitation of the cardio-augmentor nerves 

 can cause rhythmical contractions in the perfectly quiescent heart of 

 molluscs, and a sudden and prolonged outburst of beats of great 

 force in the frog's heart, which has been brought to a standstill by 

 cautiously heating it to 40 to 43 C. (Practical .Exercises, p. 194) fo'r 

 a minute or two, or to a considerably lower temperature, for a longer 

 time (Fig. 77). A similar effect can be obtained on the quiescent 

 mammalian heart by stimulation of the nervi accelerantes. 



28" 5 

 S30 



M 1 1 1 1 m 1 1 ii i 1 1 1 1 1 1 n M i 1 1 1 1 1 1 1 1 1 n 1 1 1 1 1 n rr 



jq g . 77 . Effect of Stimulation of Frog's Cardiac Sympathetic during Complete 

 Standstill of the Heart at 28-5 C. Upper tracing, auricle ; lower, ventricle. 

 To be read from right to left. Time-trace, two-second intervals. 



The Normal Excitation of the Cardiac Nervous Mechanism. 

 We have now to inquire how this elaborate nervous mechanism is 

 normally set into action. And we may say at once, that striking as 

 are the effects of experimental stimulation of the vagus trunk or the 

 nervi accelerantes in their course, it is only under exceptional cir- 

 cumstances that the efferent nerve-fibres, at any rate before they 

 have entered the heart, can be directly excited in the intact body. 

 In certain cases the pressure of a tumour or an aneurism on the 

 nerve-trunks, or, in the case of the accelerators the progress of a 

 pathological change in the sympathetic ganglia through which the 



