THE CIRCULATION OF THE BLOOD 675 



are that the auricular contraction (1) produces only a slight increase of pressure 

 in the ventricle, since the latter is quite lax. As the ventricle contracts, there 

 is a small rise of pressure in the auricle, due to pressing back of the mitral valve 

 as it closes. The ventricular pressure rises rapidly, without any increase in aortic 

 pressure, since the semilunar valves are closed by the greater pressure in the aorta. 

 As soon as the intra-ventricular pressure is slightly greater than that of the aorta 

 (at 6), these valves open and the aortic pressure rises simultaneously with the 

 further rise in the ventricular pressure, the two curves being practically parallel, 

 until the ventricle commences to relax. Since blood is flowing from ventricle to 

 aorta during this period, the pressure in the former must be somewhat higher than 

 in the latter, and we notice that it is not until the ventricular pressure has fallen 

 somewhat, at the line c, that the semilunar valves close, marked by a series of 

 vibrations on the aortic curve. The further course of the ventricular pressure 

 curve is independent of that of the aorta. The significance of the remaining 

 points marked on the curve will be found in the description of the figures. The 

 actual shape of the top of the ventricular pressure curve, during the time of 

 driving blood into the aorta, varies according to the resistance in the arterial 

 system. It may be dome-shaped or have indications of waves on it, but, on the 

 whole, it is a kind of plateau, compared with the rapid rise and fall. It appears, 

 then, that the form found by Chauveau and Marey, and confirmed by Bayliss and 

 Starling, is the correct one, although the waves are somewhat exaggerated in 

 these, especially in Chauveau and Marey's. The more or less sharp-peaked 

 curves, obtained by some investigators, are due to insufficient accuracy of 

 response of the instrumental method used. 



It is interesting to note that there is no change in length of the muscle fibres 

 of the ventricle until nearly the full height of contraction is reached. It will 

 be remembered that A. V. Hill (page 443 above) showed that the maximal external 

 work is obtained from skeletal muscle if not allowed to shorten until the full state 

 of tension is developed, so that the heart muscle works, in this respect, under 

 nearly optimal conditions. Owing to the curvature of the heart, however, it 

 is clear that only a part of the force of the contraction is exerted in the direction 

 required, namely, inwards, so that the fibres act at a considerable mechanical 

 disadvantage. 



The researches of Patterson and Starling (1914) show that the work done 

 by the heart is determined by the amount of blood flowing into the ventricles 

 from the venous side. Up to a very high rate of inflow, the power of raising this 

 volume to the aortic pressure is fully adequate. The limit at which this power fails 

 is much higher than had been supposed from previous experimental work, in which 

 sufficient venous supply was not provided. Put in another way, the energy 

 produced in a ventricular contraction is in direct proportion to the length of the 

 muscle fibres at the time when contraction begins. Thus the heart muscle obeys 

 a similar law to that of skeletal muscle, in which we saw (page 443 above) that 

 the energy developed is in relation to the magnitude of certain action surfaces 

 in the fibres. 



Patterson, Piper, and Starling (1914) show in more detail how the length of 

 the muscle fibres during contraction determines the output. The energy of each 

 systole is proportional to the preceding diastolic volume. This view is found to 

 explain all the facts. The same relationship was shown by Kozawa (1915) to hold 

 for the heart of the tortoise. 



THE HEART SOUNDS 



The heart sounds have for centuries attracted attention, chiefly owing to 

 their use in clinical diagnosis. Thos. Lewis (1913, 2), by an improvement in 

 the microphone method of Einthoven, has obtained interesting records with the 

 string galvanometer, using two parallel strings, so that the electrical change 

 of the muscle can be recorded at the same time. This addition to the instrument 

 has shown itself very valuable also in comparing the electro-cardiograms from 

 different leads. Fig. 228 gives three records, one a normal record from the 



