The anatomical treatise gives in seventeen short chapters a perfectly clear and connected account of the action of the heart and of the movement of the blood round the body in a circle. A movement which had been foreshadowed by some of the earlier anatomists and had been clearly indicated by Harvey himself as early as 1616. But it is here laid down with a precision of detail, with a logical exactness, and with a wealth of illustration which is marvellous even to us who read of the circulation as an established and fundamental principle upon which the whole body of physic rests. Harvey's proof fell short of complete demonstration, for he had no means of showing how the smallest arteries are connected with the smallest veins. He worked, indeed, with a simple lens, but its magnifying power was too feeble to show him the arterioles and the venules, whilst the idea of an injection does not seem to have occurred to him. It was not until after the invention of the compound microscope that Leeuwenhoek, in 1675, described the blood corpuscles and the circulation in the capillary blood vessels, though they had already been seen by Malpighi. The first chapter of the Treatise is introductory. It is a review of the chief theories which had been held as to the uses of the heart and lungs. It had been maintained that the heart was the great centre for the production of heat. The blood was driven alternately to and from the heart, being sucked into it during the diastole and driven from it during the systole. The use of the arteries was to fan and cool the blood, as the lungs fanned and cooled the heart, for the pulse was due to an active dilatation and contraction of the arteries. During their dilatation the arteries sucked in air, and during their contraction they discharged murky vapours through pores in the flesh and skin. In the heart, as well as in the arteries, the dilatation was of greater importance than the contraction. The whole of this tissue of falsehood seems to have been founded upon an incorrect apprehension of the nature of heat. It was looked upon as a fundamental principle or entity, and until chemistry and physics reached the stage of experimental sciences it was impossible to give a correct explanation of the phenomena it presents. Even Harvey sometimes lost himself in mysticism when he had to deal with the subject of animal heat, though he was struggling hard to find a firm foothold when he said, "We are too much in the habit of worshipping names to the neglect of things. The word Blood has nothing of grandiloquence about it, for it signifies a substance which we have before our eyes and can touch; but before such titles as Spirit and Calidum Innatum [or inherent heat] we stand agape."