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I am not sure that it is not doing more work than that now while I am 

 lecturing. In the case of the young men who pull in this race for twenty- 

 three minutes, every ounce of muscle in their arms and legs gives out a 

 force that in a minute would lift 20 lbs. through a foot. If any of you 

 have seen the exhausted condition of those young men when taken out 

 of their boats after twenty-three minutes, you will, I think, agree with 

 me that human nature could not endure such labour for forty minutes ; 

 yet the heart of an old man close upon a hundred years of age has 

 worked for that hundred years of his life as hard as the muscles of the 

 young men that pull in the Oxford and Cambridge eight-oared races. 



We have now discussed, and I hope satisfactorily solved, the ques- 

 tion how much work is done by the heart ; but a question remains un- 

 answered which no intelligent mind can avoid asking : How does the 

 heart do that work ? I cannot pretend to tell you how ultimately it 

 does that work, for that depends upon the problem of nerve-supply — 

 a subject with which we are totally unacquainted. But I believe I have 

 succeeded in making one step further in advance and getting at a slight 

 knowledge of the arrangement of the fibres of the heart by which this 

 enormous amount of work is possible, and have arrived at it by a 

 strict and rigorous application of the principle of least action. I have 

 applied the principle of least action to the construction of the heart, 

 so as to ascertain, if possible, some law that must be fulfilled by the 

 arrangement of the fibres which will allow of this principle being car- 

 ried out. The law of muscular contraction which must be complied 

 with is this : Let [l] represent the length of a muscular fibre ; an order 

 comes from the brain or some other part of the nervous system to this 

 fibre to contract ; it is immediately shortened to an extent that leaves 

 it about eight-ninths of its original length. Now, when a group of 

 fibres are so arranged, as in the example I showed you before of the 

 triangular muscle, that each fibre in the system is not at liberty to con- 

 tract to eight-ninths of its entire length, there is a necessary loss of 

 force. Therefore, if the principle of least action applied to the heart 

 be true, we must find such an arrangement of the fibres in the heart as 

 will allow of every individual fibre contracting to eight-ninths of its 

 length. The fibres of the heart have been compared by Borelli to a 

 ball of twine ; and this has been more correctly explained by subse- 

 quent writers as two balls of twine contained in a third. There are 

 two cavities in a heart ; we call them the right and left ventricles ; the 

 whole heart surrounds these. Certain groups of fibres run round one 

 cavity, certain groups run round another, and certain other groups run 

 round both. The fibres that run round the entire heart are called com- 

 mon fibres, because they are fibres which are common to both cavities, 

 while the fibres that run round each cavity separately are called proper 



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