CIKCULATION 



249 



Work of the Heart 



The heart in pumping blood through it is doing work, and 

 the amount of work may be expressed in work units e.g. kilo- 

 grammetres (see p. 60). The method employed to measure this 

 in the dog is to determine, by means of the cardiometer, the 

 amount of blood expelled at 

 each systole, and to find the 

 resistance against which it 

 is expelled. This gives the 

 factors for determining the 

 work done at each systole, 

 and it is easy to calculate 

 the total work in any given 

 period (fig. 118). 



Nature of Cardiac Contraction 



The contraction of the ven- 

 tricle lasts for a considerable 

 period 0'3 seconds. Is it of 

 the nature of a single contrac- 

 tion, or of a tetanus ? 



It is impossible to tetanise 

 heart muscle, even by rapidly 

 repeated induction shocks. 

 A single stimulus applied to 

 heart muscle produces a single prolonged contraction. Again, 

 the mode of development of the currents of action does not 

 indicate anything of the nature of a tetanus. With each beat 

 of the ventricles the variation in the electric potential begins 

 at the base and travels rapidly to the apex. This passage of the 

 contraction wave along the fibres explains the great length of 

 the ventricular systole as a whole. There can be no doubt 

 that each contraction of heart muscle is of the nature of a 

 muscle twitch. In this respect heart muscle resembles non- 

 striped muscle. 



It further resembles it in that the minimum stimulus is 

 also a maximum stimulus i.e. the smallest stimulus which 

 will make the muscle contract makes it contract to the utmost. 



FIG. 118. Roy's Cardiometer to measure 

 the output of Blood from the Heart. 



b, heart in cardiometer chamber ; 



c, piston recorder working on lever 

 against rubber band, d. 



