CAUSE OF THE HEARTS BEAT. 249 



moved from all the rest of the body, will go on beating for 

 some time; even the heart of a warm-blooded animal, if 

 supplied with oxygenated blood, may be kept beating regu- 

 larly for hours after its isolation from the rest of the body. 

 The excised heart of a turtle or frog if kept moist will beat 

 for days. Whether the time of its continuance be shorter 

 or longer, the fact that the heart-beat continues after com- 

 plete excision of the organ proves that it is not dependent 

 on stimuli originating in other parts of the Body. In the 

 ciliated cell we had no differentiation into muscle and 

 nerve its contractile and automatic parts if separated at 

 all were not optically distinguishable and we could only 

 speak of the cilia as still retaining both of those primitive 

 protoplasmic properties. But in the heart, where we find 

 distinct muscles and nerves, the question naturally arises 

 in which of them does the automatic power reside. We 

 have already seen (Chap. X.) that ordinary striated mus- 

 cles possess little or no automaticity : they only contract 

 under the influence of a recognizable stimulus, and though 

 the muscular fibres of the heart do differ somewhat from 

 other striated muscle, it is a priori improbable that they 

 are automatic and we are rather led to suppose that the 

 usual stimulus starts in the ganglion-cells of the heart, 

 especially since we know that nerve-cells elsewhere are 

 automatic. Experiment confirms this supposition. If an 

 excised beating frog's heart be cut into several pieces 

 with a sharp razor it will be found that, while bits of the 

 auricles and the base of the ventricle go on beating, the 

 apical portions of the ventricle lie at rest permanently or 

 for an hour or two not because the muscle there is dead 

 and has lost its contractility, for these bits if excited by any 

 extraneous muscular stimulus will still beat but because 

 that part of the heart possesses little automacity. Now this 

 is just the part of the frog's heart that has no ganglion-cells 

 of its own, while the parts that go on beating are those which 

 possess them; hence we conclude that the normal stimulus 

 originates in the nerve-cells of the organ. The excitant of 

 the nerve-cells being still unknown we call them automatic. 



