210 



THE CONTRACTION OF CARDIAC MUSCLE. 



and ventricle has only been observed by me in the heart of the toad or 

 frog, where, in consequence of the combination of the accelerator nerves 

 with the vagus, the diminution of contraction force has been accom- 

 panied with acceleration of rate. 



In addition to the depression of rhythmical power, of excitability, of 

 contraction force and of conductivity, the vagus produces as a primary 

 effect a depression of tonicity, so that the muscular tissue relaxes when 

 the nerve is stimulated. 



Ever since the action of the vagus has been studied by physiologists, the 

 flaccid condition of the heart distended with blood, which is seen during 

 standstill, has attracted their attention, and has been, again and again, 

 attributed to an active relaxation of the cardiac muscle due to the stimulation 

 of the inhibitory nerve. The difficulty has always been to prove that such 

 relaxation does in reality take place, owing to the difficulty already mentioned 



FIG. 116. Modification of Roy's tonometer. 



of proving that the cardiac muscle possesses tone in addition to its rhythmical 

 contraction. The extent of relaxation between the contractions depends on 

 the rapidity of rate of beat ; any influence, therefore, such as the action of the 

 vagus nerve, which slows rate, must increase the relaxation between the 

 contractions without necessarily altering the tone of the muscle. In order, 

 then, to be sure that the nerve does alter the tonicity of the muscle, the rate 

 of beat must be unaltered or even quickened at the same time that the 

 relaxation of the muscle is observed. Also it is clear that the extent of 

 relaxation can be more easily observed by the measurement of the volume 

 of the ventricle or auricle, than by any linear measurement, such as is given 

 by the method of suspension. In order to estimate this relaxation, I have 

 made use of a modified Eoy's tonometer, 1 as seen in the accompanying 

 Fig. 116. 2 



It consists essentially of two parts a glass chamber a, which contains the 

 heart and its nerve, and a recording chamber b, which contains oil, and is 



1 Roy, Journ. PhysioL, Cambridge and London, 1878, vol. i. 

 - Cf. Phil. Trans., London, op. cit., p. 1019. 



