The heart and circulation

Af Hans Kermit

In 1628 the English doctor, William Harvey, described the pumping function of the heart and the passage of blood through the arteries and veins. In 1661 Malpighi, with the help of a microscope, proved that a network of capillaries join the arterioles and venules. Stensen had read these works and used them in his anatomical works. In 1662 he carried out the vivisection of a raven, studied the heartbeat and saw how the heart was filled with venous blood from the vena cava (the great vein in the thorax), from which it was pumped out via the pulmonary artery. Afterwards he tried to see what would happen if he interfered in the blood’s circulation. In his essay on the dissection of a shark’s head he mentions a comparable exercise carried out on a dog:

I have observed that a ligature placed on the aorta, the latter not having been perforated, stopped all movement in the lower parts, every time I tightened it, with movement returning each time I loosened the ligature. I demonstrated this several years ago in Florence, where the dog survived the experiment, without any restriction on its freedom of movement, once the ligature was removed.

The experiment has been given the name of ‘Steno’s experiment’. Stensen had shown that the function of a muscle was dependent on its blood supply.

Hippocrates had asserted that the heart was a muscle and William Harvey cited him in 1628, but he had not stated his reasons for believing it to be so. No one in Stensen’s day and age took this assertion seriously: the heart was thought to be the home of the soul, a mystically superior organ. Neither Descartes nor Harvey had entirely rid themselves of these notions. To say that the heart was a muscle and nothing more, was completely unheard of.

Already during his student days in Leiden Steensen had cooked and dissected an oxe’s heart and found that its structure resembled that of muscles. He began to investigate muscular tissue in other parts of the body. Steensen published his findings in De musculis et glandulis observationum specimen (Observations of muscles and glands). He explains at length the structure of muscles and how the contractions take place in the muscle’s fibres and not, as previously thought, in the sinews. His study of muscle structure showed that the more fibrous structure of the heart was nevertheless the same as the other muscles he had examined. ‘Yes, truly, the heart is a muscle’ he exclaimed, as a sort of affirmation of Hippocrates and William Harvey. He continued:

The heart cannot, therefore, be a special substance, not the seat of heat, of innate warmth, of the soul. It cannot even produce its own fluid as blood, nor can it bring forth spirits, as for example the vital spirit.

Had Stensen’s contemporaries accepted this theory, they would have had to abandon many of their opinions  regarding the function of the body. As a result, his discovery was not generally accepted.