The most valuable discovery made by Stensen was undoubtedly that the heart is a muscle. It {147} must not be forgotten that in his time, Harvey's discovery of the circulation of the blood was not yet generally accepted; indeed, there were many who considered the theory (as they called it) of the English investigator as one of the passing fads of medicine. Two significant discoveries, made after Harvey, served, however, to establish the theory of the circulation of the blood on a firm basis and to make it a definite medical doctrine. The most important of these was Malpighi's discovery that the capillaries--that is, the minute vessels at the end of the arterial tree on the surface of the body and in various organs--served as the direct connexion between the veins and the arteries. This demonstrated just how the blood passed from the arterial to the venous system. Scarcely less important, however, for the confirmation of Harvey's teaching was Stensen's demonstration of the muscular character of the tissue of the heart. Some of his observations upon muscles are extremely interesting, and, though he made many mistakes in explaining their function, he added not a little to the anatomical and physiological knowledge of the time in their regard. He seems to have been one of the first to recognize the fact that in the higher animals the heart may continue to beat for a considerable time after the animal is apparently dead; and, indeed, that by irritation of the removed heart, voluntary contractions may be brought about which will continue spontaneously for some moments. {148} With regard to the objections made by some, that such studies as these upon muscles could scarcely be expected to produce any direct result for the treatment of disease, or in the ordinary practice of medicine, Stensen said in reply that it is only on the basis of the anatomical, physiological, and pathological observation that progress in medicine is to be looked for. In spite, then, of the discouragement of the many, who look always for immediate practical results, Stensen continued his investigation, and even proposed to make an extended study of the mechanism of the muscular action. In the meantime, however, there had gradually been coming into his life another element which was to prove more absorbing than even his zeal for scientific discovery. It is this which constitutes the essential index of the man's character and has been sadly misunderstood by many of his biographers. Sir Michael Foster, of Cambridge, England, in his "Lectures on the History of Physiology," originally delivered as the Lane Lectures at Cooper Medical College, San Francisco, said:--