THE NERVOUS REGULATION OF THE HEART 



165 



muscle gains a certain amount of support from the facts that these 

 drugs act very much in the same way on the heart of the mammalian 

 embryo (rat, rabbit, etc.) before and after the development of its in- 

 trinsic nervous system, and that the % passage of an interrupted current 

 through the heart of very young embryos causes distinct inhibition. 

 But, as has already been pointed out, it is not legitimate to transfer 

 without question to the muscle of the fully developed heart the proper- 

 ties of the embryonic cardiac tissue. And, on the other hand, musca-^ 

 rine fails to affect the heart in many invertebrate animals for instance, 

 in 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 substances on the 

 three heart-tissues (gang- 

 lion, motor nerve plexus, 

 and muscle) is one of 

 degree only (Meek). 



Stannius ' Experiment. 

 Another series of pheno- 

 mena, 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 classical 

 experiment we have al- 

 ready mentioned (p. 144), 

 and they are also described 

 in the Practical Exercises 

 (p. 192). They are easy 

 to verify, but difficult to 

 interpret. The most prob- 

 able explanation of the Fig 76- _ Frog < s Heart. Sympathetic stimulated 

 standstill caused by the (30 n^. between the coils). Temperature 12. 

 first ligature is that the Marked increase in force. Only auricular tracing 

 lower portion of the heart, reproduced. Time-trace, two-second intervals, 

 when cut off from the 



sinus in which the beat normally originates, needs some time for the 

 development of its automatic power to the point at which an indepen- 

 dent rhythm can be maintained. The effects following the second 

 Stannius ligature seem to depend upon the power of the ventricle to 

 develop and maintain an independent rhythm, but the contractions are 

 supposed by some to be started by stimulation of the muscular tissue 

 in the auriculo- ventricular groove by the ligature. 



Nature of Inhibition and Augmentation. So far we have been dis- 

 cussing 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 or two bring it to a standstill. 

 The question cannot fail to press itself upon the mind of anyone who 

 has ever witnessed this most beautiful of physiological experiments; 

 but as yet there is no answer except ingenious speculations. The most 

 plausible of these is the trophic theory of Gaskell, who sees in the vagus 



