LECTURE lll.— Tuesday, June dth, 1871. 



Application of the principle of Least Action to the Heart and other in- 

 voluntary Muscles. — The Mechanism of the Heart explained, and the 

 amonnt of work done by it. — '-^ Experimenttun crticis^^ of the entire 

 Theory, derived from the measurements of the Fibres of the Heart of 

 Man and the Ox. — General conclusions as to the future progress of 

 Aniinal Mechanics and Comparative Anatomy, when subject to the 

 Ride of Geometry, the Queen and Mistress of all the Sciences. 



I HAVE reserved for my closing lecture to day the most wonderful and 

 remarkable of all the examples I am able to give you of the application 

 of the principle of least action to animal mechanics. It relates to a 

 question which deeply interests every person in this room ; it relates to 

 the action of our hearts. It is my intention to endeavour to lay before 

 you the wo;k which is done by our hearts, and the manner in which 

 those hearts do their work. The story of the heart is a most wonderful 

 and mysterious story, and you must make allowance for the difiiculty of 

 the subject, and the defects of the lecturer, if I fail to convey perfectly 

 to your minds all that is in my own mind respecting it. It is not easy 

 to condense into one short hour the results of a labour of ten years. 

 The progress of discovery is slow, and it is difficult to explain to those 

 who have not been travelling in the same paths of research as myself, 

 all the meaning and the bearings of the facts which I have to state ; 

 you will therefore, I hope, excuse me if occasionally you fail to see the 

 connecting link that joins one part of my reasoning with another, and 

 take for granted that if I had a longer time at my disposal, or more 

 art in the mode of laying my materials before you, I could make you 

 perfectly understand all that I know with regard to this subject. 



We have first to consider the question of the amount of work which 

 is done by our hearts. The heart is a small muscle weighing only a few 

 ounces, and it beats perpetually day and night, summer and winter. 

 Frequently an old man's heart approaching a hundred years of age will 

 be found on examination as perfect a mechanism and as complete as it 

 was when he was a young man of twenty. In order to measure the 

 force and power of the human heart, the most obvious method that 

 would suggest itself is one that is impossible to adopt, because it would 

 require the death of the person on whom the experiment was made, 

 \Ve have experimented on the hearts of horses, oxen, sheep, dogs, and other 



