142 A MANUAL OF PHYSIOLOGY 



keeps the heart in a state of permanent inhibition, which is 

 removed when atropia cuts out the nerve-endings. It is 

 quite in accordance with this, that muscarine has no effect 

 on a heart whose vagus nerves, as occasionally happens, 

 have no inhibitory power. 



Some observers have supposed that although muscarine and pilo- 

 carpine in large doses do act on the nervous structures of the sinus, 

 their primary and chief effect is to depress the rhythmical power of 

 the muscle, which atropia, on the other hand, increases (Gaskell). 

 And this view gains a certain amount of support from the facts that 

 muscarine and atropia act very much in the same way on the heart of 

 the mammalian embryo (rat, rabbit, etc.) before and after the develop- 

 ment of its intrinsic nervous system, and that the passage of an 

 interrupted current through the heart of very young embryos causes 

 distinct inhibition. But, on the other hand, muscarine fails to affect 

 the heart in many invertebrate animals for instance, in the Daphnia 

 (Pickering). So that the only conclusion to which it is possible to 

 come is that we do not as yet thoroughly understand either the mode 

 of action of these substances or their point of attack. 



Stannius' Experiment. Nor can much more be said of another 

 series of phenomena that are intimately related to our present subject, 

 and have excited, since they were first made known by Stannius, an 

 enormous amount of discussion. The chief facts of this classical 

 experiment we have already mentioned (p. 130), and they are also 

 described in the 'Practical Exercises' (p. 175). They are easy to 

 verify, but difficult to interpret. To Gaskell and his followers the 

 most probable 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 normally originates, needs some time for the develop- 

 ment of its rhythmical power to the point at which an independent 

 rhythm can be maintained. For in the heart of the tortoise, in which 

 a similar temporary standstill of the auricles and ventricle occurs 

 when the former are detached from the sinus, the circulation of a 

 blood solution through the coronary vessels or the application of 

 atropia, both of which, according to Gaskell, increase the rhythmical 

 power of the cardiac muscle, prevents or removes the standstill. The 

 effects following the second Stannius ligature are supposed to be due 

 to stimulation of the muscular tissue by the ligature. But it is not 

 easy to explain why the second ligature should stimulate the ventricle 

 in preference to the auricles, and why the first ligature should 

 apparently not stimulate the muscular tissue at all. Nor does the 

 explanation become easier if we suppose, as is sometimes done, that 

 it is the Bidder's ganglia which are stimulated by the ligature or by the 

 knife, for there is no real evidence that they have motor functions. 



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 



