376 The Law of the Heart [May 20, 



heart or of voluntary muscle, shows that these are composed of 

 innumerable fibrils, so that internally the muscle is made up of struc- 

 tures presenting an enormous extension of longitudinal surfaces. 

 The more the muscle is stretched, the greater will be the extent of 

 these surfaces. A large amount of evidence, based on the electrical 

 and chemical changes occurring in muscle as a result of excitation, 

 points to the contraction as being essentially a surface phenomenon — 

 .a molecular change over the whole of the longitudinal surface which 

 may result in a polarisation or depolarisation of the surface and an 

 increase of surface tension, so that the muscle is a surface tension 

 machine in which there is on excitation a direct conversion of 

 chemical into surface energy. The greater the surface the greater 

 will be the number of molecules involved, so that increased length 

 of muscle must increase at the same time the total chemical changes 

 and the total tension produced by the summation of the surface 

 tension of each fibril. 



It is only by such a change of molecular dimensions that we can 

 explain the rapidity of events in a muscle (the insect wing muscle 

 •can contract and relax 300 times per second), or the high efficiency 

 of the machine, an efficiency which A. Y. Hill has shown may 

 amount to 100 per cent for each isolated contraction, and over a 

 length of time to 50 per cent. As directly measured in the heart- 

 lung preparation, we find a mechanical efficiency of about 25 to 30 

 per cent. 



Conclusion, 



It is impossible here to enter into the applications of this law of 

 the heart, but so far it has not failed in accounting for the behaviour 

 •of this organ under all manner of conditions, either in health or 

 disease. It is important to remember, however, that we are dealing 

 Tiere with the isolated heart. In the natural body the mechanisms 

 which we have studied are fenced round, protected and aided by the 

 complex activity of the central nervous system, which is always acting 

 on the heart, balancing its activity against that of the blood vessels, 

 and co-ordinating it with the events which are occurring in every 

 other part of the body. Mi these factors must be taken into account 

 when we are endeavouring to form a conception of the total behaviour 

 of this organ under the varying activities of the intact animal. 



[E. H. S.] 



