THE CAUSATION OF THE HEART-BEAT 947 



firming for this special case the general view of inhibition long ago put forward by 

 Morat, but not now generally accepted. 



The heart muscle does not show a refractory period, but on direct stimulation with 

 repeated shocks there may be a summation of contractions, which may fuse to a com- 

 plete tetanus. The question naturally arises how far the heart of Limulus is to be 

 regarded as a special case, or how far we may transfer results gained from experience 

 on this heart to those of other hearts in which a perfect separation between ganglion- 

 cells and muscle fibres is not so easily attainable. Carlson has sought to show the 

 applicability of his results to the explanation of the cardiac mechanism in vertebrates 

 by a series of observations on other invertebrates' hearts, where the muscular and 

 nervous tissues are not so easily dissociable. Such hearts present phenomena very 

 analogous to those of the frog's heart. According to him the phenomenon of the re- 

 fractory period, the * all or none ' law of contraction, and the absence of tetanus in the 

 heart of the frog is due, not to the peculiar functions of the muscle fibres, but to the 

 fact that in all our experiments we are affecting muscular and nervous tissues 

 si mult aneously . 



In the absence of more perfect knowledge of the properties of the nerve-nets which 

 surround involuntary and cardiac muscle fibres a decision of the point is not yet possible. 

 The muscle and nerve fibres of Limulus show, however, important differences from the 

 cardiac muscle of the frog in their reaction to chemical stimuli. Acceptation of the 

 neurogenic theory would necessitate the prediction of a type of nervous tissue endowed 

 with properties for which we have no analogy in any of the nerve tissues which have 

 been the subject of exact investigation, whereas the myogenic theory only ascribes to 

 the muscleicells of the heart properties which are the common attribute of all protoplasm 

 or are displayed in a less marked degree by the ordinary skeletal muscle fibres. It 

 would, at any rate, be premature to transfer unreservedly all the results obtained on the 

 heart of the Limulus, the muscle fibres of which have the structure and behaviour of 

 skeletal muscle fibres, to the explanation of the phenomena exhibited by the hearts of 

 vertebrates. 



THE HEART-BEAT AS A WAVE OF CONTRACTION 



It the ibeat of the frog's ventricle, or a strip of mammalian ventricle, be 

 recorded, the curve obtained resembles closely the twitch of a voluntary 

 muscle produced in response to a single excitation. Whereas, however, a 

 single contraction with the subsequent relaxation of voluntary muscle only 

 lasts about one-tenth of a second, the contraction of the mammalian ventri- 

 cular muscle lasts three-tenths of a second, of the frog's ventricle about 

 half a second, and of the tortoise ventricle about two seconds.* In the heart, 

 as in a voluntary muscle fibre, the contractile process originates at the 

 stimulated point and travels thence to all other points. 



The progress of the excitatory wave is well seen if a record be taken of the 

 electrical changes resulting in the frog's heart from a single stimulation. If 

 the two ends of a strip of ventricular muscle be connected with the two 

 terminals of a capillary electrometer, stimulation at one end causes a diphasic 

 variation, showing that the excitatory process starts at the stimulated end 

 and travels to the other end of the heart. Thus if the acid of the electro- 

 meter be connected with the base of the ventricle and the mercury of the 

 capillary be connected with the apex, stimulation at the. base causes a wave 

 passing from base to apex. Directly after the stimulation therefore the base 



* The duration of the contraction depends on the temperature. The figures given 

 are for the mammalian heart at 37 C. and for the amphibian heart at about. 15 C. 



