THE CIRCULATION OF THE BLOOD AND LYMPH 151 



legitimate to transfer without question to the muscle of the fully 

 developed heart the properties of the embryonic cardiac tissue. 

 And, on the other hand, muscarine fails to affect the heart in many 

 invertebrate animals for instance, in the Daphnia (Pickering). 

 Yet it is probable that, while the various tissues in the heart possess 

 a different susceptibility to one and the same drug, if the dose is 

 large enough it may affect them all. In the Limulus heart, where 

 the question can be most easily tested, it has been found that the 

 selective action of alkaloids, anaesthetics, and various other sub- 

 stances on the three heart-tissues (ganglion, motor nerve plexus, 

 and muscle) is one of degree only (Meek). 



Stannius' Experiment. An other series of phenomena, intimately 

 related to our present subject, have excited, since they were first 

 made known by Stannius, an enormous amount of discussion. The 

 chief facts of this classi- 

 cal experiment we have 

 already mentioned (p. 

 132), and they are also 

 described in the Prac- 

 tical Exercises (p. 183). 

 They are easy to verify, 

 but difficult to inter- 

 pret. The most prob- 

 abb explanation of the 

 standstill caused by 

 the first ligature is 

 that the lower portion 

 of the heart, when cut 

 off from the sinus in 

 which the beat nor- 

 mally originates, needs 

 some time for the de- 

 velopment of its auto- 

 matic power to the 

 point at which an in- 

 dependent rhythm can 

 be maintained. The F IG - 64. FROG'S HEART. 



effects following the Sympathetic stimulated (30 mm. between the 

 second Stannius liga- coils). Temperature 12. Marked increase in force, 

 ture are supposed by Only auricular tracing reproduced. Time -trace, 

 some to be due to two-second intervals. ; 

 stimulation of the mus- 

 cular tissue in the auriculo-ventricular grooveTjby the ligature. 



Another view is that the first ligature stimulates the inhibitory 

 mechanism (vagus fibres) at the junction of the sinus and right 

 auricle, a position in which it is specially sensitive to stimuli. This 

 causes inhibition of the whole of the heart below the ligature. The 

 second ligature cuts off the ventricle from the inhibitory impulses, 

 while leaving the auricle still under their influence. The Stannius 

 experiment does not succeed in the mammalian heart or, at any 

 rate, only imperfectly. 



Nature of Inhibition and Augmentation. So far we have been 

 discussing the phenomena of inhibition and augmentation as ultimate 

 facts. We have not attempted to go behind them, nor to ask what 

 it is that really happens when inhibitory impulses fall into a heart, 

 which from the first days of embryonic life has gone on beating with 

 a regular rhythm, and in the space of a second 01 two bring it to a 



