1Messieurs,

2As we said in the last session, we will now consider the life of beings as stemming from universal life. Life will be studied within each species or rather each class. Thereby constrained, physiology is even of greater interest.

3When we examine a living body, at a state so close to its origin that our senses cannot make any distinction, the only things we will see, whether for birds or mammals, are semi-fluid molecules, with a shape, but a shape different from the animal it will become. As it grows, the first recognizable features develop, the primitive form becomes more complex, the protruding parts sprout —they do not add themselves on the exterior like minerals. Further growth occurs the same way, from inside out even if the external molecules enter inside the body through different ways. The introduction of new external molecules determines the exit, through excrement, perspiration or exhalation, of previous ones. For a while, all motions are combined in a way that the quantity of output is lower than the inputs. This is when the animal grows in all directions. Soon, growth slows down or occurs differently. In the latter case, the incoming molecules only increase the density of parts on a fixed surface. Then comes a time when all motion ceases, the links between the parts of the living body also cease to function, the body molecules are abandoned at the usual action of elements. Fermentation, putrefaction, or other motion of this type dissolve and separate the different molecules. Some go back to their aqueous state, others spread as gas in the atmosphere, others, of a more fixed nature, fall like the caput mortuum1 inside the earth.

4This is the general assessment of the constituent phenomena of life. You see that life is only a series of more or less accelerated motions but the existence of a living body has to be assumed otherwise they could not happen. Life, I repeat, always requires a body. However, it is not impossible that inside the living body, the universal motion producing the secretions of so many various parts produces the sprout of a new life. This is the major question of generation that we will discuss later. We will now focus on life itself, the machine or being in which vital motions occur, the diversity and complexity of these motions, and the underlying forces. There were varying opinions on these forces. We will examine them first because the driving causes are the most essential part of physiology.

5During the history of this science in the seventeenth century, we saw that a specific school derived from Descartes’s system2 led by Sylvius, or Le Boë,3 explained the physiological phenomena using chemistry doctrines. This school recognized fermentation, resulting from the acidity of some fluids and the alkalinity of others. The combination of fluids with specific chemical proprieties was their explanation of heat production, secretions, etc. De Graaf,4 Tachenius,5 and other physicians adopted this system and tried to adapt it to medicine, but soon realized it was based on fanciful assumptions. Two men in particular refuted the system through their research on animal and plant fluids: Bohnius,6 a professor at the Faculty of Leipzig, and Boerhaave,7 who demonstrated that neither acids nor alkalis existed in bodies in which they presumably existed and that they only developed after life as the result of fermentation. Therefore, the chemistry system did not have a great number of followers in the eighteenth century. It is only recalled by Vieussens,8 a professor in Montpellier, whom I will mention when I will talk about the anatomy of the brain and the nervous system. This author, in his Traité nouveau de la structure du cœur (Treatise on the Structure of the Heart), published in Toulouse in 1715,9 attributed blood circulation to a fermentation caused by the nitrous principle of arterial blood and the sulfurous principle of venous blood.

6The main prevailing systems were the mathematical system, the psychological system —under which all bodily actions were attributed to the soul— and the volatility system, later called irritability. These three systems successively prevailed. The first one started in Italy, at Borelli’s school10 and ended with Boerhaave’s physiology. The principle of archeus11 that first appeared in Van Helmont’s12 works led to Stahl’s13 psychological doctrine, and after some changes, became the vital principle of Sauvages.14 The third one, irritability, appeared in Glisson’s15 book, then in the works of Friedrich Hoffmann16 and Gorter;17 it was significantly developed by Haller,18 who overly focused on irritability and neglected the influence on temperament of the nerves disseminated in muscular parts. Other physiologists reproduced this system, now the dominating one. We will examine more thoroughly than last year the works of the authors supporting these three systems.

7Iatro-mathematicians19 were philosophers who tried to apply the rules of mechanics to the functioning of the body and assessed, from a mathematical perspective, the forces behind the functions. As we saw, the first was Alfonso Borelli,20 a pupil of Galileo.21 His treatise De motu animalium22 includes his research on the forces of motion in animals. He recognized that in order to move a light weight, nature gave the muscles at the origin of the motion strength far greater than the weight itself, as this force was at a disadvantage. His book focuses almost entirely on this demonstration, at the time a novelty but trivial today. To explain the action of muscle fibers, he recognized some sort of fermentation from the combination of air and blood in the lungs, making the fluids circulate in abundance in the muscle fibers. According to him, these fibers are a series of vesicles that swell and shorten due to the influx of fluid caused by fermentation. Secretions were explained by the size of the vessels but he also recognized the presence of ferment. His theory is in part chemical and in part mathematical.

8In his analysis of muscle forces, as soon as he produces data obtained by applying the lever theory, his results are impossible to demonstrate. To establish the force of the heart, he compares its weight to another muscle, for example the biceps, the chief flexor of the arm. He estimates the force required for this muscle to lift a given weight and establishes a relation concluding that the equilibrium strength of the heart would be one hundred and thirty-five thousand pounds. This reasoning is not accurate as two muscles can have different forces and still be able to lift the same weight. Furthermore, all the estimations by iatro-mathematicians changed significantly. Their results proved the lack of means to find the truth in the type of research they carried out.

9Borelli, as we saw, mixed the chemical and the mathematical systems but also took some elements from the cognitive system, saying that motion could be caused by willpower and habit. The same applies to the chest, also influenced by air. But this comparison is not quite right because we cannot stop heart motion on our own will power while we can hold our breath.

10Then comes Pitcarne,23 a professor in Leiden and one of Boerhaave’s masters and colleagues.24 For Pitcarne, life depends on circulation. This is not an absolute truth because many animals do not have circulation organs.

11According to Pitcarne, animal heat results from friction of the blood moving inside vessels. Vital force comes purely from the heart and animal faculty results from a specific secretion of blood in the brain. This secretion creates the nervous fluid setting off the nerves, which in turn move the muscles. According to Pitcarne, air does not mix with blood. Breathing only serves to facilitate blood circulation through the lungs. As the vascular system broadens with the splitting of vessels —the joining of small vessels being larger than the big stems at the origin— circulation slows down in the former, the air going through the lungs presses the blood in the small vessels and causes all secretions. The same pressure in the atmosphere is used by Pitcarne to explain the transformation in the lungs of venous blood into arterial blood.

12Lorenzo Bellini,25 another physician from Borelli’s school,26 thinks like Pitcarne that the slowing down of blood in the small arteries causes the changes in molecules, or secretions. With Borelli, he assumes that the contraction of muscle fibers is the result of an influx of fluids that abruptly become scarce. He thinks the heart is the core agent of these motions.

13James Keill27 was born in Edinburgh in 1673 and died in 1719. He taught anatomy in Oxford and Cambridge and adopted the ideas of the iatro-mathematicians. In a book titled Tentamina medico-physica28 printed in 1708, he determined that the weight of blood in a one hundred and sixty-pound body is one hundred pounds, an exaggeration. He estimated that blood has a velocity of five thousand two hundred and thirty-three feet per minute in the aorta and only one foot in the small capillaries.

14In another treatise,29 Keill attempted to estimate the force of the heart and weighed the column of blood this muscle can lift. Here is how he proceeded and the result he obtained: he adapted a glass tube to an artery and observed the height reached by blood before stabilizing. He weighed the blood column corresponding to the force of the heart and determined it to be about five ounces. This experiment shows that the force of the heart is reduced to almost nothing. Borelli’s result of one hundred and thirty-five thousands was exaggerated. Keill’s was also but in the opposite way.

15This physician-mathematician also explained secretion by the slowing down of blood in small arteries and muscular motion by the swelling of vesicles caused by the influx of nervous fluid. This system that considers muscle fibers to be hollow dominated during almost the first half of the eighteenth century. It was only more recently that it was discovered that muscle fibers meander and shorten.

16James Jurin,30 an Englishman like Keill, was born in London in 1680 and died in 1750. He was the president of the College of Physicians and a member of the London Royal Society. He also worked on the force of the heart.31 His results were different from Keill’s. He determined that the blood column supported by the left ventricle weighed nine ounces and the one supported by the right ventricle six ounces. His calculations on blood velocity also differed from Keill’s and others’ from the same school. Such divergences discredited the application of mathematics to physiology. All authors we mentioned attempted to achieve in life physics some advances similar to what geometricians managed for ordinary physics and astronomy. Through their discoveries, Galileo,32 Kepler,33 Newton34 focused the minds on geometry and it is only normal that physicians tried to do the same for their science. The application of mathematics was suitable for astronomy because this science is extremely simple, the motion of the stars occurs in a void without any resistance.35 Because the attraction of small stars, only recently assessed by Laplace,36 can be neglected, the calculation of planetary motion is almost restricted to the combination of the gravitational pull of the sun and tangential load. But the difficulty is insurmountable to estimate a liquid motion within a hollow muscle that cannot be measured from a geometric point of view. The liquid continues to flow through countless vessels in varying directions. To solve such a complicated problem, methods far superior to the ones existing in geometry today are needed. The efforts of physicians to reach the same progress in physiology by applying mathematics, as was the case for physics, optics, or astronomy, were useless. The same difficulties and errors are found in the works of Baglivi,37 a professor in Rome who was born in 1688 and died prematurely in 1707, exhausted from his works. His work De fibra motrice specimen,38 printed in 1700 has an advantage over everything else we have reviewed. It demonstrates the action of solids on vital phenomena. Baglivi was the leader of the solidist medical sect.39 For him, the dura mater40 was a propelling organ opposing the heart. This is a mistake because the dura mater does not have contractile fibers and completely adheres to the skull.

17Dominico Santorini,41 a native of Venice, was born in 1681 and died in 1737. He also worked on applying mathematics to physiology. The main idea of his work De structura et motu fibrae,42 printed in Venice in 1705, was that each fiber is formed of an extended nervous thread. This opinion was not successful because there is an essential difference between fibers and nerves.

18We will now focus on a man who should be recognized for his vast knowledge. We already talked about Hermann Boerhaave43 as a chemist and a botanist. We will now focus on his work as a physiologist. You know he was born in Voorhout in 1688. His father, a pastor, gave him his first instruction. At the age of eleven, he mastered Latin and Greek. An incurable ulcer he caught at a very young age gave him the idea of becoming a physician for self-treatment. He started his studies in Leiden in 1681 and became a doctor in Harderwick44 in 1693. In 1701, he replaced Drelincourt,45 a professor at the University of Leiden and his main medicine master as the dean. In 1709, he became a clinical professor and in 1718 a chemistry professor. He combined these three chairs to the chair of physiology and held them for the rest of his life. No professor of his time was as eloquent and as famous as a physician. His reputation was such that his consultations were sought from people coming from all over the world. His fortune rapidly increased and exceeded four million. He used it in the noblest way.

19Usually, physiologists simultaneously tackle study of physiology and research on anatomy. Boerhaave did not do both and performed very few dissections. However he would go each year to Amsterdam to study Ruysch’s46 anatomical discoveries and acquired knowledge of the structure of the human body. Strangely, Boerhaave adopted Malpighi’s47 ideas on secretions rather than Ruysch’s.

20Boerhaave’s great merit was to complete the refutation of chemical hypotheses. Because of its style and clarity, his book Institutiones rei medicae,48 with a first edition published in 1708, served for a long time as the foundation of public lectures in Europe. Its intelligibility was such that it was even translated in Arabic for Turkish schools where his main ideas on inflammation were adopted up to a point. According to him, inflammation is determined by blood flow in lymph vessels under the influence of excited nervous action. Boerhaave attempted to explain secretions by the shape of ducts, the variation of their diameter and how more or less difficult blood molecules were distributed. However, these ideas are far too mechanical.

21In addition, Boerhaave presented many ideas that became popular on the action of parties. We will see these opinions later, developed by the famous Haller,49 his pupil. We will finish this review of physicians linked to the mathematical school with some minor authors.

22We will first cite Bernoulli,50 a professor in Basel and then in Groningen. At the time, chairs were selected at random and it happened that a philosopher would get the chair of medicine and a geometrician the chair of history. It is said that it was because of this usage that he wrote his thesis De motu musculorum et fermentatione,51 in which he recognized that nervous minds emit blood from fibers through fermentation that produces the swelling of fibrous vesicles. He made many calculations in this respect and developed an entire system. The Bernoulli family was very well known for their mathematical works but did not leave anything remarkable in physiology.

23I will only name Michelotti52 and Pacchioni,53 both physicians from Borelli’s school, to whom we will come back under anatomy. I will also only mention Hales,54 the author of the Haemastatics.55 We will come back to him under the subject of plant physiology. The works of other iatro-mathematicians are only repetitions of those we already analyzed.

24We will now review the psychological system. The living body is obviously ruled by laws distinct from those regulating ordinary mineral bodies. From the instant life ceases, the body changes its state, dissolves, and turns into various molecules. The question was whether the specific laws of the living body are linked to its structure or to a principle outside this structure that dominates it. In one word, what is the nature of the principle of the living body? Van Helmont56 believed that molecules, or elements comprising the body, were held together by a special principle, that this principle dominated and led their motions in a way to maintain their order and even to restore such order when it was only partially destroyed. To support his opinion, he mentioned what has been called since, the “thorn of Van Helmont.”57 Everyone knows that when a foreign body penetrates the skin, the motions of the body immediately change as the fluids head to the point of irritation; an inflammation occurs, resulting in festering. Festering produces the isolation and expulsion of the irritating body. The wound rapidly heals and the original order is reestablished. This recurs for thousands of illnesses. Van Helmont called archeus58 the cause of this phenomenon that is beyond our control. He assumed that it was half-material half-spiritual and regarded it as the preservative of the body.

25It is obvious that this does not mean anything. An occult principle, an unexplained principle that gives a clear understanding of the details of phenomena, has to be distinguished from a general principle, to which any name is given and does not explain anything. This distinction is essential in the philosophy of the science of living beings and I will develop my thought further. Please give me a moment of attention.

26Authors who recognized that an archeus, a reasonable soul, or a vital principle could explain the phenomena of a living body claim to follow the example of astronomers who use universal gravitation as the principle of the movement of the stars, even if they cannot explain the cause of gravitation. This reasoning would be correct if physiological phenomena result as clearly from the presumed principle astronomical phenomena from the principle of gravity.

27However, it was admitted that universal gravity is exerted on bodies in direct relationship with their masses and in inverse relationship with the square of their distance. Accordingly, given the mass of bodies, the distance between them, and the motion exerted on each one, the motion of these bodies can be determined by calculation in eternity, provided that no disruptive masses occur. This meets the intelligence: phenomena receive a rational and mathematical explanation. There is no ulterior cause to search for to prove their existence because it will only explain the cause of the cause, which would be curious but not essential.

28Do the archeus, the reasonable soul, the vital principle explain physiological phenomena as satisfactorily as gravitation explains planetary motions? Do they dispense us of secondary causes in the same way gravitation does not require ulterior explanation? Can we apply their vague generality to specific phenomena, either by calculation, or by some argument? Not in the least. Claiming like Van Helmont that ordinary and extraordinary body motions are produced by a specific principle he called archeus is saying one word, expressing an abstract idea using an abstract term. To satisfy intelligence, the proprieties of this archeus should be explained and it should be shown how they could explain all organic movements. There was nothing as such.

29Therefore, Van Helmont’s ideas were lost for a while. Chemists attempted to explain body motion through fermentation, mathematicians through hydrostatics and all assumed that the body was formed in a certain way, that there was a certain mechanism that had a cause beyond their research. They all avoided using the abstract and general principle of archeus.

30Psychological or Stahlian physiology59 was produced by Descartes’s system60 under which bodies were considered to be incapable of self-motion and only the mind had the power of activation. This system was the opposite of the one supported by Leibniz61 —he did not invent it— that each monad had its own force, a certain energy with which it can activate the motion of other monads. Leibniz’s system had followers like Stahl’s, and we will see F. Hoffmann62 deriving a doctrine under which he attributes their own energy to living bodies, the cause of their regular motions. Stahl was no more than Leibniz the inventor of the fundamental idea of his system. It was Borelli63 who said, as you might recall, that heart motion could result from will power and habit. Stahl’s system is so substantial that it would be impossible for me to completely present it today. We will recount its history and its developments in the next session.