Yet Harvey was only able to demonstrate it by experiments performed on the living bodies of frogs and deer. Since Harvey's time, the laws of the circulation have been established with admirable precision. Hales demonstrated the pressure of blood in the vessels. Chauveau and Marey introduced into the heart of a horse an apparatus which enabled the pressure of the blood in the heart, in the arteries, and in the veins, to be measured. Weber found that the pneumogastric nerve stopped the heart's action. Ludwig applied the graphic method to the circulation. Delicate instruments have been constructed which give diagrams of the pulsations and measure the pressure of the blood in the arteries and in the heart of man. Claude Bernard discovered the nerves which regulate the movements of the vessel walls. In short, the whole history of the circulation is due solely to vivisections, and it would be ridiculous to speak of our uncertainties in this respect; for the essential mechanical or nervous laws of the circulation are as well known now as those of the combinations of nitrogen with oxygen. 2. _The Respiration_ remained profoundly unknown, as to its inmost nature, right up to Lavoisier's time. Lavoisier placed some guinea-pigs in a box filled with ice, measured the quantity of heat thrown off, the quantity of oxygen consumed, the quantity of carbonic acid produced; and he was thus able to deduce a fundamental law of life, viz., that life is essentially combustion. He made experiments on himself also; but however great one's respect for the life of a guinea-pig may be, must it be considered wrong that Lavoisier should have experimented on the guinea-pig before experimenting on himself? As for the laws which regulate this consumption of oxygen and this production of carbonic acid, to discover these it was necessary to put into cages animals of every species and of every size. And there is, perhaps, not a single physiologist who has not made this experiment, at the risk of annoying the cats and dogs thus exposed--without, as far as that goes, doing them any harm--to varied temperatures or to different diets. Moreover, in order to study the respiratory exchanges, physiologists experiment on man as well; is, therefore, the extraordinary scruple against experimenting on animals to be imposed upon them also?