THE CIRCULATION OF THE BLOOD AND LYMPH 131 



muscular tissue in other parts of the vascular system, e.g., 

 in the central artery of the rabbit's ear, and the veins of the 

 bat's wing. At the same time it must be remembered that 

 full and formal proof of the myogenic origin of the cardiac 

 beat has not yet been given. It is probable but not proven. 

 It must also be borne in mind that when we have localized 

 the essential mechanism of the rhythmical contraction in 

 the muscle of the heart, we have still to ask whether this 

 mechanism is not put into action by some stimulus external to 

 the substance of the muscle. And, indeed, a certain amount 

 of evidence has been brought forward that the normal stimu- 

 lus (for the heart of the terrapin) depends on the presence of 

 calcium and potassium salts in the blood (Howell). 



We have seen that there is a normal order or sequence in which 

 the different parts of the heart contract, the contraction beginning 

 both in the frog and in the mammal at the base, and travelling more 

 or less rapidly towards the apex. It would seem that the muscular 

 tissue of the part of the heart in which the beat begins has a higher 

 rhythmical power than the rest of the cardiac muscle, and that 

 normally the contraction is only propagated, not originated, by the 

 lower portion of the heart. But under certain conditions the normal 

 sequence can be reversed. In the heart of the skate, it is easy by 

 stimulating the bulbus arteriosus to cause a contraction passing from 

 bulbus to sinus. Not only may the normal sequence be changed in 

 the entire heart, but any part of the heart may apparently have its 

 rhythmical power exalted by appropriate means, so that it can be 

 brought to beat rhythmically when isolated from the rest of the heart. 

 On the other hand, the power of propagating the contraction may be 

 artificially interfered with increased by heat, diminished by cold, 

 abolished by pressure or fatigue. If, e.g., a frog's heart is supported 

 by a clamp fixed in the auriculo-ventricular groove, and the clamp is 

 tightened or the ventricle cooled, while the auricle is at the ordinary 

 temperature, or if the auricle is heated while the ventricle is at the 

 ordinary temperature, only every second or third auricular beat will 

 be followed by a ventricular beat (p. 172). 



In addition to its marked power of rhythmical contraction, 

 the cardiac muscle is distinguished from ordinary skeletal 

 muscle by other peculiarities. The most striking of these is 

 that * it is everything or nothing with the heart ' ; in other 

 words, the heart muscle, when it contracts, makes the best 

 effort of which it is capable at the time ; a weak stimulus, if 

 it can just produce a beat, causes as great a contraction as 

 a strong stimulus. Another peculiarity is that a true tetanus 

 of the cardiac muscle cannot be obtained at all, or only under 



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