990 PHYSIOLOGY 



regarded ax 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 

 refractory 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 inultaneously . 



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

 surround in voluntary 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 predication 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 ascribes only to 

 the muscle cells 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 



If the beat 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 lasts 

 only 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. 1 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 

 becomes negative and the column of mercury moves towards the acid; 

 a moment later the contraction extends to the apex. All parts of the 

 heart are now in a similar condition of excitation : there is no difference of 

 potential between the two terminals, and the mercury comes back quickly 



1 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. 



