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animals. The first of these series of experiments was made by the 

 celebrated Dr. Hales at the close of the last century : it consisted in 

 measuring, by direct experiment with tubes, the amount of the hydro- 

 statical pressure inside the cavities of the hearts of several animals. 

 These experiments showed that the hydrostatical pressure inside the 

 hearts of animals varies ; in the horse and ox and larger animals 

 amounting to a pressure of nine feet perpendicular of fluid blood, and 

 in the smaller animals to somewhat less. From these experiments we 

 can calculate without much difficulty the total amount of work which 

 is done by the heart of a horse, the heart of an ox, the heart of a sheep, 

 or the heart of a dog ; but you will see it would be impossible to per- 

 form such an experiment upon our own hearts, because the experiment 

 is necessarily accompanied with the death of the animal that is operated 

 upon. We can calculate from these experiments also what I call the 

 coefficient of capillary resistance. The heart pumps the blood through 

 the large arteries of the body into the capillary vessels which permeate 

 every tissue in our frames, and the great resistance to the action of the 

 heart occurs in forcing the blood through these capillary vessels. I 

 have placed before you here, as the result of direct experiment, the co- 

 efficient of capillary resistance of the sheep, y^g- ; the dog, y^-g ; the 

 horse, g-^-g-; the ox, -g^-g. You observe in the sheep and dog the co- 

 efficients are double what they are in the horse and ox. These 

 animals group themselves naturally together into the smaller animals 

 with a double coefficient of resistance, and the larger animals with a 

 single coefficient. Now with which of these groups of animals are we 

 to associate ourselves in making a calculation as to the amount of work 

 which is performed daily by our hearts ? As I explained before, we 

 cannot perform direct experiments upon the human subject ; but an 

 accident placed in my power the means of making a very close ap- 

 proximation to this remarkable result. When the artery of a horse or 

 of a cow is cut, we can measure with ease the height to which the 

 blood will spout into the air ; and when the experiment is made, we 

 are surprised at first to find that the artery does not spout to the height 

 of nine feet. We can prove that there is an hydrostatical pressure in- 

 side the heart of the horse amounting to a nine feet column of blood, 

 but when the artery is cut the blood will only spout to a height of two 

 and a half feet. Nature making instinctively a spontaneous effort to 

 shut off the pressure to save the animal from death by bleeding. If, 

 therefore, we could by any process arrive at the precise height to which 

 the blood would spout from our arteries if wounded, and compare them 

 with the corresponding experiment in the sheep, the horse, and the ox, 

 we should find which of these groups of animals man is most closely 

 allied to with regard to the circulation of his blood. 



On the 1 8th of March, in the year 1863, I witnessed an operation in 



