The Italian microscopist Marcello Malpighi (1628-1694) was the first to see the capillaries and was a founder of histology, embryology, plant anatomy, and comparative anatomy.
On March 10, 1628, Marcello Malpighi was born at Crevalcore near Bologna. He attended the University of Bologna, where he graduated in philosophy and in medicine in 1653. Malpighi became a lecturer in logic at Bologna in 1655 but left in 1656 to be professor of theoretical medicine at Pisa. There he met Giovanni Borelli, a mathematician who had recently turned his attention to the analysis of movement in animals.
Malpighi returned to Bologna in 1659, where he was made extraordinary lecturer in theoretical medicine. Through Borelli's influence, Malpighi was elected to the first chair in medicine at Messina in 1662, but in 1666 he returned to Bologna to become professor of medicine, and he remained there for the next 25 years.
By 1667 Malpighi's work had already aroused the interest of the recently formed Royal Society in London, and one of its secretaries wrote to him suggesting that he communicate his results to the society. Malpighi responded favorably, and most of his later books were published in London. He was elected a foreign member of the Royal Society in 1668.
Malpighi was also a successful physician, and in 1691 he became the personal physician of Pope Innocent XII in Rome. Malpighi died on Nov. 29, 1694, in Rome.
Discovery of Capillaries
In September 1660 Malpighi began to study the structure of the lungs, and within nine months he had communicated the results of these studies in two letters to Borelli in Pisa, who published them under the title De pulmonibus observationes anatomicae (1661). Malpighi presented "a few little observations that might increase the things found out about the lungs." These observations included the first descriptions of the air sacs (pulmonary alveoli) in the lungs of a dog and of the pulmonary capillaries in the frog and tortoise.
Having convinced himself of the presence of direct connections between the arteries and veins in the lungs of the frog and the tortoise, Malpighi was prepared to speculate that the same was so in other animals: he was unable to see such anastomoses in the dog's lung, perhaps, because these "small vessels escape the senses on account of their smallness."
The science of the study of the structure of tissues was established by the classical microscopists, and Malpighi's contributions were among the most important. He published four tracts in 1665. The first one described the presence of "red globules of fat" in the blood vessels of the mesentery of the hedgehog. This is one of the earliest descriptions of the red blood cell, although Malpighi did not realize the significance of his observation. In other tracts he described the papillae of the tongue and the skin and suggested that these may have a sensory function. The layer of cells in the skin now known as the Malpighian layer was also described. The last tract of 1665 concerned the general structure of the brain. Malpighi showed that the white matter consists of bundles of fibers which connect the brain with the spinal cord. He described the gray nuclei that occur in the white matter.
Malpighi's De viscerum structura execitatio anatomica (1666) gives a detailed and fairly accurate account of the structure of the liver, spleen, and kidney. Malpighi dissected the tissue under the microscope, and he identified small particulate masses or "lobules," resembling bunches of grapes, in the liver. Each lobule was composed of "tiny conglobate bodies like grape seeds" connected by central vessels. Having seen these lobules in the livers of several species, he concluded that the lobule was the fundamental hepatic unit. He believed that the lobules were supplied by fine blood vessels and that their function was secretory. Malpighi realized, therefore, that one function of the liver is as a gland and that the bile duct must be the passage through which the secreted material (bile) passes: the gall-bladder was, therefore, not the site of origin of bile. He was also able to prove in an animal experiment that the gallbladder is only a temporary store for bile on its way to the intestine. Malpighi speculated that bile might be useful in the process of digestion.
Although Malpighi was fond of describing many structures as "glands," he realized, from his study of the blood supply to the spleen, that this organ is not a gland but a contractile vascular organ. He was the first to describe the lymphatic bodies (Malpighian corpuscles) in the spleen.
Whereas other anatomists believed that the outermost part of the kidney was structureless, Malpighi showed that it is composed of many little wormlike vessels (the renal tubules) which he called "canaliculi." Although he could not demonstrate any continuity between the convoluted canaliculi and the straight tubules in the central mass of tissue (medulla), he predicted that such a continuity exists. Malpighi's description of how he discovered the glomeruli in the outer part of the kidney is vivid: "In all kidneys which up to this time I have been able to get, I have detected a number of very small glands [that is, Malpighian corpuscles, or glomeruli]. In order to see these glands, black fluid mixed with spirit of wine should be injected through the renal artery. And when the kidney is sectione…. one will see these same innumerable glands attached like apples to the blood vessels, the latter swollen with the black liquid and stretched out into the form of a beautiful tree." Malpighi realized that the "glands" were connected to the "extreme ends of the arteries" and to the veins but did not observe the true nature of the "glands," that is, that they are composed of a tuft of capillaries. He speculated that their function was to secrete the urine.
Malpighi's detailed description of the medulla of the kidney showed how the canaliculi converge on the pelvis and enter the ureter. In pathological specimens he observed the formation of kidney stones in the pelvis.
Malpighi's memoir De bombyce (1669), on the silk-worm moth, was the first detailed account of the structure of an invertebrate. Prior to his study, it was still believed that such small creatures were devoid of internal organs, and he himself was surprised to find that the moth was just as complex as higher animals. He not only discovered the trachae and spiracles, the system of tubes and holes through which insects breathe, but also correctly guessed their function. He was the first to describe the nerve cord and ganglia, the silk glands, the multichambered heart, and the urinary tubules, which still bear his name.
With his microscope, Malpighi was able to study much earlier stages of the embryo than had hitherto been possible. His results were communicated to the Royal Society in two memoirs: De formatione pulli in ovo (1672) and De ovo incubato (1675), which placed embryological study on a firm basis of sound observation. He saw the heart within 30 hours of incubation and noticed that it began to beat before the blood reddened. He described the development of the dorsal folds, the brain, the mesoblastic somites, and structures which were later identified as gill arches. However, Malpighi believed that he had seen the form of an embryo in an unincubated egg. A possible explanation is that the egg, being 2 days old, had been "incubated" in the hot Italian August sun. This observation was used, not by Malpighi himself as much as by his followers, to support the doctrine of preformation, that is, that the whole adult was present in the egg and had only to be "unfolded" by a suitable stimulus.
Some of Malpighi's most extensive writing, beautifully illustrated, is on plants. Malpighi and his contemporary Nehemiah Grew became the confounders of plant anatomy by their systematic studies on the microscopic structure of plants. Malpighi's book Anatome plantarum was published in two parts in 1675 and in 1679. His illustrations frequently show the plant cell with its wall, first described by Robert Hooke in 1665.
Malpighi's interest in the structure of plants began when he noticed the broken branch of a chestnut tree which had fine threads projecting from the surface. Upon examining these with his lens he was struck with their resemblance to the air tubes of insects. Although he wrongly concluded that they served the function of breathing, his enthusiasm for the study of plants had been awakened. His drawings of the stems of higher plants distinguished between the annular rings of the dicotyledon and the scattered bundles of the monocotyledon (the terms dicotyledon and monocotyledon were introduced in the early 18th century). He suggested that material required for growth of the plant was formed from the sap by the leaves, but the erroneous idea that the sap circulated, as blood did, was originated by other writers.
Malpighi's work on the development of plants is just as significant as that on the development of animals. He made drawings of the embryo sac and endosperm and gave a superb account of the germination of seeds in which he differentiated between those later called monocotyledons and dicotyledons. He was the first to describe tubercles on leguminous roots, and he showed that some galls contained a grub. He traced the grub back to an egg and onward to an insect, of which he illustrated the egg-laying apparatus.
Further Reading on Marcello Malpighi
Some biographical information on Malpighi is in Howard B. Adelmann, Marcello Malpighi and the Evolution of Embryology (1966), and in Circulation of the Blood: Men and Ideas, edited by Alfred P. Fishman and Dickinson W. Richards (1964). For background see Charles Singer, A History of Biology to about the Year 1900 (1931; 3d rev. ed. 1959).
Additional Biography Sources
Malpighi, Marcello, The correspondence of Marcello Malpighi, Ithaca N.Y. Cornell University Press, 1975.