1066 PHYSIOLOGY 



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 

 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 only ascribes to the muscle-cells of 

 the heart properties which are the common attribute of all protoplasm, or are dis- 

 played 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 only lasts about one-tenth of a second, the contrac- 

 tion of the mammalian ventricular muscle lasts three-tenths to four- 

 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 volun- 

 tary muscle fibre, the contracting power 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 heart from a single stimula- 

 tion. 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 electrometer 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 to the base line. Relaxation, like contraction, 

 starts first at the base and proceeds thence to the apex. There is thus 

 a small period during which the apex is still contracted while the 

 base is relaxed and the apex is therefore negative to the base. This 

 terminal negativity of the apex is shown on the capillary electrometer 

 by the excursion of the column of mercury away from the point of 

 the capillary (cp. Fig. 88, p. 259). 



