The broad points in connection with the vascular system being thus settled, Harvey turned his attention more particularly to the mechanism of the heart's action. He shows that the two auricles move synchronously and that the two ventricles also contract at the same time. Hitherto it had been supposed that each cavity of the heart moved independently, so that every cardiac cycle consisted of four distinct movements. To prove that the movement of the heart was double he examined the eel, several fish, and some of the higher animals. He noticed that the ventricles would pulsate without the auricles, and that if the heart were cut into several pieces "the several parts may still be seen contracting and relaxing." The minute accuracy of Harvey's observation is shown by his record of what is in reality a perfusion experiment. He says: "Experimenting with a pigeon upon one occasion after the heart had wholly ceased to pulsate and the auricles too had become motionless, I kept my finger wetted with saliva and warm for a short time upon the heart and noticed that under the influence of this fomentation it recovered new strength and life, so that both ventricles and auricles pulsated, contracting and relaxing alternately, recalled as it were from death to life." We now know that this was due to the warmth, to the moisture, and to the alkalinity of Harvey's saliva, so that he performed crudely, and no doubt by accident, one of the most modern experiments to show that the heart, under suitable conditions, has the power of recovering from fatigue. This portion of the treatise affords an insight into the enormous amount of labour which Harvey had expended in its production, for he says: "I have also observed that nearly all animals have truly a heart, not the larger creatures only and those that have red blood, but the smaller and pale-blooded ones also, such as slugs, snails, scallops, shrimps, crabs, crayfish, and many others; nay, even in wasps, hornets, and flies I have, with the aid of a magnifying glass and at the upper part of what is called the tail, both seen the heart pulsating and shown it to many others." That this was the result of a careful study of the animals mentioned and not a simple observation is shown by the following sentences: "In winter and the colder season, pale-blooded animals such as the snail show no pulsations: they seem rather to live after the manner of vegetables or of those other productions which are therefore designated plant animals.... We have a small shrimp in these countries, which is taken in the Thames and in the sea, the whole of whose body is transparent: this creature, placed in a little water, has frequently afforded myself and particular friends an opportunity of observing the movements of the heart with the greatest distinctness, the external parts of the body presenting no obstacle to our view, but the heart being perceived as though it had been seen through a window.