1 66 



THE CIRCULATION OF THE BLOOD AND LYMPH 



a nerve which so acts upon the chemical changes going on in the heart 

 as to give them a trophic, or anabolic, or constructive turn, and thus to 

 lessen for the time the destructive changes underlying the muscular 

 contraction. The augmentor nerves, on the other hand, are supposed 

 to exert a katabolic influence, and to favour these destructive changes. 

 And while, according to Gaskell, the natural consequence of inhibition 

 is a stage of increased efficiency and working power when the inhibition 

 has passed away, the natural complement of augmentation is a tem- 

 porary exhaustion. But it must be remembered that this distinction 

 is not as yet based upon any very solid foundation of actually observed 

 and easily interpreted facts. 



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 rhyth- 

 mical 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. 192) for a minute or 

 two, or to a consider- 

 ably lower tempera- 

 ture, for a longer time 

 (Fig. 77). A similar 

 effect can be obtained 

 on the quiescent mam- 

 malian heart by stimu- 

 lation of the nervi 

 accelerantes. 



28 



~m 



Fig. 77. Effect of Stimulation of Frog's Cardiac Sym- 

 pathetic 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 Exci- 

 tation of the Cardiac 

 Nervous Mechanism. 

 We have now to in- 

 quire 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 



