"But in some of the pale-blooded and colder animals, as in snails, whelks, shrimps, and shell-fish, there is a part which pulsates--a kind of vesicule or auricle without a heart--slowly, indeed, and not to be perceived except in the warmer season of the year.... In fishes, serpents, lizards, tortoises, frogs, and others of the same kind there is a heart present, furnished with both an auricle and a ventricle.... And then in regard to animals that are yet larger and warmer and more perfect,... these require a larger, stronger, and more fleshy heart.... Every animal that has lungs has two ventricles to its heart, one right, the other left, and whenever there is a right there is a left ventricle, but the contrary does not hold good; where there is a left there is not always a right ventricle.... It is to be observed, however, that all this is otherwise in the embryo where there is not such a difference between the two ventricles.... Both ventricles also have the same office to perform, whence their equality of constitution. It is only when the lungs come to be used ... that the difference in point of strength and other things between the two ventricles becomes apparent. In the altered circumstances the right has only to drive the blood through the lungs, whilst the left has to propel it through the whole body." This concludes Harvey's Demonstration of the Circulation of the Blood in 1628, but he continued to work at the subject throughout his life. In two letters or anatomical disquisitions, addressed to the younger Riolanus of Paris, and dated from Cambridge in 1649, Harvey gives his latest reflections upon the subject of the Circulation of the Blood. These disquisitions differ very greatly from the original treatise. They are less clear and concise, and dwell more upon points of dispute which had arisen in connection with the controversy, which raged for many years round Harvey's discovery. The first disquisition is devoted more especially to the question of the anastomosis which takes place between the arteries and the veins, whilst the second disquisition illustrates more fully a number of details connected with the nature and quantity of the blood and its mode of progression. Harvey says incorrectly of the anastomosis, "Neither in the liver, spleen, lungs, kidneys, nor any other viscus, is such a thing as an anastomosis to be seen, and by boiling I have rendered the whole parenchyma of these organs so friable that it could be shaken like dust from the fibres or picked away with a needle, until I could trace the fibres or every sub-division, and see every capillary filament distinctly. I can, therefore, boldly affirm that there is neither any anastomosis of the vena portæ with the cava, of the arteries with the veins, or of the capillary ramifications of the biliary ducts, which can be traced through the entire liver, with the veins."