The various histological details being thus settled there came a long interval until chemistry was sufficiently advanced to enable definite statements to be made about the aëration of the blood. The work of Black in 1757 and of Priestley and others in 1774 and 1775 at last allowed the process of respiration and the true function of the lungs to be explained upon scientific grounds. But the interval between the discovery of the capillaries and the explanation of the act of respiration was not wholly barren; for in 1732 Archdeacon Hales, by means of experiments, obtained an important insight into the hydraulics of the circulation. During the present century our knowledge of the physics of the heart and circulation has been reduced almost to an exact science by the labours of the German, French, and Cambridge schools of physiology under the guidance respectively of Ludwig, of Chauveau, and of Foster; whilst the nervous mechanism of the heart and of the arteries has been thoroughly investigated by Gaskell and others. CHAPTER IX THE TREATISE ON DEVELOPMENT Fuller, speaking of Harvey, says very ingeniously: "The Doctor though living a Bachelor, may be said to have left three hopeful sons to posterity: his books, "1. De circulatione sanguinis, which I may call his son and heir: the Doctor living to see it at full age and generally received. "2. De generatione, as yet in its minority: but I assure you growing up apace into public credit. "3. De ovo, as yet in the nonage thereof; but infants may be men in due time." The treatises on Development are so full of detail that it is impossible to give an exact notion of their contents in a popular work. They contain however certain passages of personal and of general interest which must not be omitted.