Physiology

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carried on was in many respects wrong as, for instance, his idea that the collection of facts should be allotted to one set of men, and the drawing conclusions from the facts to another. In the great majority of cases, the discoverer of a new law of nature has to find his facts for himself. Nevertheless, Bacon's works gave a great impulse to the new method. Even those who had made discoveries without knowing anything of what Bacon had written were gratified and encouraged by learning from his works that they had, without distinct consciousness of it, been treading in the path of true philosophy; while all scientific workers were helped by being able to quote in support of their methods of enquiry his incisive and illuminating words.

In spite of its success in the case of Anatomy, the new method was slow in laying hold of Physiology. More than half a century passed before it did so; but, when in 1628 William Harvey (1578-1657), by the publication of his Exercitatio de Motu Cordis, shattered the Galenic Physiology as completely as Vesalius had put an end to the Galenic Anatomy, the effects were profound and far-reaching.

According to Galen, the crude venous blood, enriched in the liver by the food brought from the alimentary canal and endowed with the nutritive qualities spoken of as the "natural spirits," flows from the heart to all parts of the body along the veins, and returns back to the heart along the same channels. Some of this blood, passing from the right side of the heart to the left by minute pores in the septum of the ventricles, invisible to the eye of man, mixes there with air sucked in through the lungs by the action of the heart. This mixture is by the innate heat of the heart "concocted" into arterial blood, endowed with "vital spirits" which flows from the heart along the arteries to all parts of the body, returning by the same channels. When it reaches the ventricles of the brain, the arterial blood, by the help of air drawn in through the pores of the ethmoid, or sieve-like, bone, gives rise to "animal spirits,” and, flowing as a pure spirit along the nerves, carries out sensation and movement.

Long before Harvey, Michael Servetus (1511-53), in his Restitutio Christianismi, published in 1553, but written long before, used words showing that he rejected wholly the supposed passage of some blood through the septum, as to which Vesalius had simply hinted his doubts, and, further, that he had grasped the true features of the pulmonary circulation, the passage of all the blood from the right side of the heart through the lungs to the left side. The same truth was taught by Colombus (1516–59), Vesalius' pupil and successor at Padua ; but there are some reasons for thinking that he had read Servetus' book. Andreas Caesalpinus (1519-1603), botanist, physician, and polemic philosopher, more than anatomist or physiologist, also enunciated views which, at all events, show that he grasped the truths both of the systemic and pulmonary circulations, the flow to the tissues along the arteries,

the return from the tissues to the heart by the veins, and the passage of blood through the lungs from the right to the left side of the heart.

Thus the doctrines of Galen had been attacked before Harvey's time; but how little effect had been produced by these attacks is shown by the teachings of Fabricius of Aquapendente (1537-1619), who, at the close of the sixteenth century, in the chair at Padua once held by Vesalius, was, by the fame of his learning, drawing students from all parts of the world, among them William Harvey. Although Fabricius did more than anyone after Vesalius to advance Anatomy, to the end of his days he taught almost pure Galenic doctrines of the circulation, ignoring what Servetus and Caesalpinus had written, and refusing to see the real meaning of his own great discovery of the valves in the veins. It was a mixed teaching, then, of new Anatomy and old Physiology which Harvey got from Fabricius at Padua, while he studied there from 1597 to 1602. But he probably learnt much outside the anatomical theatre; for during those years Galileo Galilei was working at and teaching the new Mechanics and Physics in Padua. It was perhaps partly at least through Galileo's influence that Harvey was led to apply the experimental method to Physiology, and to "give his mind to vivisections."

His first observations, as happens in many a progress, led him into a slough of despond; he began to think that "the motion of the heart was only to be comprehended by God." But the patient study of that motion in the hearts of many living animals convinced him that Galen was wrong in regarding the heart as mainly an organ of suction, and that it was, on the contrary, essentially an organ of propulsion, inasmuch as its work consisted, not in sucking air from the lungs, but in driving blood by its contractile power through the body. This new view-dimly, but only dimly, seen by Caesalpinus-led Harvey at once to a true conception of the work of the auricles and ventricles with their respective valves, and thus to the wholly new idea that all, and not some only, of the contents of the right ventricle were discharged into the lungs and so found their way to the left side of the heart. This step led to another. Making observations to determine the quantity of blood discharged at each beat from the left ventricle, and noting the frequency of the beats, he saw that the blood driven along the arteries must find its way somehow into the veins and so return to the right side of the heart. Thus, by experiments and quantitative observations, working not after the manner of Fabricius but after that of Galileo, he reached a new view of the circulation of the blood, "of a motion as it were in a circle."

And all his further observations confirmed this view, which, once fairly grasped, rendered intelligible various phenomena of the heart and blood vessels, as indeed of the body at large, hitherto obscure, and made plain the uses of those valves of the veins over which Fabricius had stumbled. It is worthy of notice that Harvey says nothing about the "spirits," so prominent in the

Harvey's successors

727 old Galenic doctrines. In an early passage he incidentally refers to them and dismisses them as irrelevant; and, indeed, though the names were used long afterwards, his teaching was their death-blow. The new conception of the same blood flowing continuously through the whole body, undergoing changes all along the circle, did away with any need for them, and at the same time rendered possible true ideas of the nutrition of the body.

Freed, as it were, by the work of Harvey from the bonds of the Galenic doctrines, Physiology expanded rapidly in the succeeding years, developing in several more or less independent directions. Its progress was greatly helped by three things: by the rapid advance of mechanical and physical learning, by the invention of the compound microscope, and, somewhat later, by the growth of a serious, no longer fantastical, Chemistry. Harvey himself, though as we have seen he seems to have been guided by the methods of the Italian physicists, made little direct use of their results. He had no microscope to help him, and his unassisted eye failed to learn how the blood passed from the small visible arteries to the small visible veins in the lungs and in the rest of the body; he could only say, it passed "somehow." There is in his books hardly a word of Chemistry, and, in his much later treatise on generation, such Chemistry as he makes use of is of the ancient kind. "He did not care for chemists," says Aubrey, "and was wont to speak against them with an undervalue."

His successors, however, fruitfully availed themselves of these aids. The compound microscope and the new Mechanics were soon made use of. In 1661, in a letter to Borelli on the structure of the lungs studied with the help of the compound microscope, Marcello Malpighi (1628-94) announced his discovery of minute channels, the capillaries, joining the ends of the pulmonary arteries to the beginnings of the pulmonary veins. These were more clearly seen by John Swammerdam (1637-80), and, even still more clearly, in the tail of the tadpole by Antony Leeuwenhoek (1632-1723); and, in a short time, this closed mode of junction of arteries and veins was found to obtain all over the body. Swammerdam, moreover, in 1658, and Leeuwenhoek in 1668, had discovered the red blood corpuscles, observed also, but at first not understood, by Malpighi. Richard Lower (1631-91) and Giovanni Alfonso Borelli (1608-97) applied to the physical problems of the circulation such as the work done by the heart, the velocity of the flow in the blood vessels, and the pressure exerted on the vascular walls—the new exact mechanical and physical learning in so complete a manner as to bring the knowledge of the subject very much to the condition in which it was when, nearly two centuries later, Poisseuille and Weber took up the same problems again. And Jean Pecquet's (1624-74) discovery, in 1651, of the thoracic duct discharging its contents into the veins of the neck, and his proof that the lacteals, discovered in 1622 by Gaspar Aselli, passed, not-as Aselli

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Boyle.

Hooke.

Mayow

thought and as suited Galenic doctrines to the liver, but to the recepta culum chyli, and so to the thoracic duct and the venous system, together with the descriptions by Olaus Rudbeck in 1653, and by others, of the vasa serosa or aquosa, that is of the general lymphatic system of the body, seemed to make the story of the flow of nutritive fluids in the body for the time complete.

In the Galenic doctrines, the use of the air in breathing was to temper the great innate heat of the heart and to provide for the escape of the "fuliginous vapours" generated during the formation of the vital spirits. Fabricius, who making use of the new mechanical learning was able to give a fairly good and correct account of the mechanics of breathing, still held to the old Galenic idea as to the use of the inspired air. The acceptance of the Harveian teaching entailed some change in this old idea, but Harvey himself was silent about it. The first step towards the truth was taken by Robert Boyle (1627-91), who, unlike Harvey, had attached himself with zeal to the rising chemical learning. In his New Experiments Physico-Mechanical touching the Air (1660), he showed that in air rarefied by the air-pump flame was extinguished and at the same time life (the life of a mouse) came to an end; thus proving that the action of the air in breathing, so far from being a tempering of heat, was on the contrary to be compared to a favouring of combustion as the source of heat. Then Robert Hooke (1635-1702) in 1667, in an experiment made before the Royal Society, showed that an animal, a dog, could be kept alive, in the absence of all movements of the chest or indeed of the lungs, by artificial respiration. He thus proved the essential feature of breathing to be the exposure of the venous blood brought by the pulmonary artery to fresh air, and that the movements of the chest were merely the means of providing repeated supplies of fresh air. This important conclusion was followed up by Lower, who in 1669 showed that the difference between venous and arterial blood, as indicated by the difference in colour, was not profound, as the Galenic doctrine supposed, but transitory, and due to the mere exposure of the venous blood to the fresh air, and to the taking-up by the blood of some of the air during the exposure; thus he was able, by giving air, to turn the purple venous blood into bright red arterial blood, and, by keeping air away, to effect the converse change.

Neither Lower nor Borelli, who had treated very fully of breathing and had also come to the conclusion that air is taken up by the blood in the lungs, nor, again, Hooke, refers to the possibility of a part of the air only being taken up. John Mayow (1643-79), who busied himself much with chemical matters, had in 1688 come to the conclusion that the air consists in part "of a certain vital, fiery, and in the highest degree fermentative spirit" which he called spiritus nitro-aëreus or igneo-aëreus. It is clear that in his spiritus nitro-aëreus Mayow had formed a conception of what more than a hundred years afterwards came to be called oxygen.

And in a tract on respiration, besides giving an admirable account of the mechanics of respiration, he showed that the air which is taken up by the venous blood in the lungs during its change into arterial blood is not the whole air, but the nitro-aerial part of it, that is the oxygen. Thus building on the foundation laid by their countryman Harvey, a small knot of Englishmen constructed almost the whole edifice of the theory of breathing.

One effect of Harvey's new teaching was a demand for more exact knowledge of the finer structure and nature of the parts through which the blood was continually flowing; for the views of the older anatomists on this matter were very vague: what was not visible fibre and blood vessels they were content to call parenchyma, and any small soft part they spoke of as a gland. This demand Malpighi was, with the help of the microscope, one of the first and one of the most successful to supply. By a series of remarkable researches on the viscera, he laid the foundation of that knowledge of the tissues which forms so large a part of modern physiology. Making use of the discoveries of the ducts of pancreatic and salivary glands by Wirsung, Wharton and Stensen, and the works of Sylvius on the features of glands, he studied the kidney, the liver and the spleen. He expounded the structure of the kidney, going far beyond the initial discovery by Bellini of the uriniferous tubules. He showed that the substance of the liver was not, as Glisson had taught, a uniform parenchyma disposed irregularly between the blood vessels, but was arranged in minute masses, which he called acini, after the fashion of the salivary and other glands; that the liver was, in fact, a huge secreting gland, secreting bile through the gall duct. And he proved that the spleen was not a gland at all but a contractile vascular organ. Thus incidentally, at one stroke, he demolished the old idea. of the liver concocting two kinds of bile, the lighter yellow, and the heavier black bile, the scum and faer as it were of fermentation, the former being discharged into the intestine and the latter going to the spleen. He turned his microscope also to other parts of the body, to the skin, the tongue, the uterus, the brain, horns, hairs, bones, and the scattered lymphatic glands; he showed that each part had a definite texture or structure, special to itself; and, though the idea of "the tissues" did not come into use until long afterwards, he was on its track. He complained that the acini of the liver were so minute that their finer "structure" could not be laid bare by the very best microscope. One cannot help fancying that, with a more powerful instrument at his command, he might have been led to a knowledge of the hepatic cell and so to the cellular constitution of the organs which he studied. Other observers also, notably Leeuwenhoek, applied the microscope to the study of the structure of parts of the body; but none went so far as Malpighi.

The progress of Physiology in another direction is so closely inter