150 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



distinguishes the remainder of the heart. This view is further supported by 

 the observation thai a slight stimulus applied to the base of a resting ventricle 

 will often provoke a series of contractions, while the same stimulus applied to 

 th«' apex will cause but ;i single contraction. 



Much may be hoped from comparative studies. In the medusas, for ex- 

 ample, the margin of the swimming bell, by the rhythmical contraction of 

 which the animal is driven through the water, is provided with a double 

 nerve-ring and ganglion-cells, while the centre contains only scattered and 

 infrequent ganglion-cells. \i' the margin is separated from the centre and 

 both arc placed in sea-water, only the pari containing many nerve-cells beats 

 rhythmically. Loeb concludes that inasmuch as the whole medusa (Gonione- 

 iiiiis) beats in sea-water in the rhythm of the margin, the failure of the iso- 

 lated centre to beat in that medium can only be explained by the lack of 

 nerve-cells. 1 



The fact that the normal contraction begins in the sinus, Howell explains 

 by the greater sensitiveness of that part to chemical stimulation. 2 



The action of muscarin on the heart is often held to indicate the nervous 

 origin of the heart-beat. Muscarin arrests the heart of the frog and other 

 vertebrates, but has no similar action on any other muscle either striped or 

 smooth, nor does it arrest the heart of insects and mollusks. It follows that 

 muscarin does not cause arrest by acting directly upon the contractile material 

 of the heart. The contractile material being excluded, the assumption of a 

 nervous mechanism on the integrity of which the heart-beat depends seems 

 necessary to explain the effect of the poison. 



Further arguments are based on uncertain analogies between the heart and 

 other rhythmically contracting organs. 



Muscular Theory of Heart-beat? — The evidence just stated cannot be re- 

 garded as proof of the nervous origin of the heart-beat. The most that can 

 be claimed is that it makes such a conception plausible. The cause of the 

 beat probably lies in the contractile substance rather than the nerve-cells. 

 It is. at all events, certain that the cardiac muscle is capable of prolonged 

 rhythmic contraction. It has been shown that a strip of muscle cut from the 

 apex of the tortoise ventricle and suspended in a moist chamber begins in a 

 few hours to beat apparently of its own accord with a regular but slow 

 rhythm, which has been seen to continue as long as thirty hours. If the strip 

 i- cut into pieces and placed on moistened glass slides, each piece will contract 

 rhythmically. Vet in the apex of the heart no nerve-cells have been found. 



The apex of the batrachian heart will beat rhythmically in response to a 

 constant stimulus. Thus if the apex is suspended in normal saline solution 

 and a constant electrical current kept passing through it, beats will appear 

 after a time, the frequency of pulsation increasing with the strength of the 



1 Loeb: American Journal of Physiology, L900, iii. p. 383. 

 » Howell: Ibid., ii. p. 47. 



3 A valuable bibliography is given by Engelmann: Archiv fur die gesammte Physiologic, 

 1896, lxv. p. lO'.t; see also Ibid., p. 035. 



