History.

27

the history

gy.

HISTORY.

THE early history of physiology can be little more Utility of than an account of the opinions of the ancient philosoof physiolo- phers respecting the nature and functions of the human body; but as their opinions reflect considerable light on the progressive improvement of the philosophy of man, the history of physiology, even in its early stages, is curious and interesting. Opinions of Of the origin of physiology as a science we know noPythagoras thing. On examining the writings of the earliest philorespecting sophers, we meet with little more than a collection of

man.

28

29

Of Alcma.

on.

abstract principles and hypothetical reasonings, especially previous to Pythagoras. He considered man as a microcosm, or an epitome of the universe, in which were produced the same phenomena as in the larger world, only to a less extent. He admitted more than one intelligent principle, conducting all the operations of the human body, He supposed that the human soul, nourished by the blood, fixed by the veins, the arteries, and the nerves, as so many visible situations, became obedient to the general laws of universal harmony. He did not pretend that the eternal power of numbers had prescribed all the phenomena of nature, and that the force of numerical harmonies regulated the motions of the bodies which filled the universe, though he has been made so to express himself by his disciples. He was contented with asserting, that every thing in nature was brought about according to the qualities and proportions of numbers, without attributing to them an intrinsic virtue and a positive existence. He perceived that the phenomena of the animal economy succeeded each other with a strict regularity, by which they concurred in maintaining order; and in this order he found the principle of the existence and preservation of all beings; a principle without which they could not exist. He considered the souls of men as emanations from the general soul of the universe, or anima mundi.

Alemæon considered the brain as the seat of the soul. He supposed sound to be produced by the reverberation of the air within the cavity of the ear; and he thought that taste was owing to the moisture of the tongue. He compared the body of a foetus to a sponge, which obtained its nourishment by the suction established over every part of its substance. According to him, the motion of the blood was the essential principle of life; and he supposed that the stagnation of this fluid in the veins produced sleep, and its active expansion brought back the waking state of the body. Health consisted in the equilibrium and well proportioned mixture of certain primary qualities; and that whenever any of these became too predominant, disease was the consequence. Of EmpeEmpedocles involved himself in a multitude of abdocles and surd hypotheses, in order to explain the formation of Anaxago- man, and the combination of the elements from which he was produced. He too, like the disciples of Pythagoras, sought among the properties of numbers, for the general principles both of physical and moral science. In uniformity with this system it was, that he reckoned the four elements, and admitted among the particles of these material principles a kind of affection and aversion, of desire and antipathy, capable of separating and reuniting them, as occasion might require. He believed that respiration commenced within the uterus, where the infant was provided from each parent with

ras.

30

certain organic particles, which tended to unite into one History. uniform whole. Anaxagoras, convinced that we must attribute the arrangement of matter to the intelligence of a superintending being, imagined that the body of every animal was formed of homogeneous particles, which were brought together by a sort of affinity. It appeared to him, that bodies which were endowed with thought, were composed of sensible elements; that these elements remained unalterable, and that no power in nature could exert any action on them.

31

Democritus dedicated his life to repeated experiments Democrion plants and animals. He explained the principal phe- tus. nomena of organized bodies by the action and reaction of atoms, which he supposed to be endowed with powers essentially active, and susceptible of repelling and attracting each other. According to him, generation consisted in the cohesion of homogeneous atoms. He conceived the heat inherent in the elements of the body to be the sole active principle with which man was animated; and that by increase of this he became capable of life and motion. He compared the organs of the senses to mirrors, on which were painted the images of things, and he reduced all sensations to the sense of feeling, which he supposed to be more or less delicate according to circumstances.

28

These philosophers, who lived before the time of Hip-Hippocra pocrates, had, as we see, but very rude and indistinct tus. notions of the animal economy; nor were those of the great father of physic much superior. Excellent as was his practice, and acute his knowledge of the symptoms and progress of diseases, the physiology of Hippocrates was very lame and defective. He seems, indeed, to have understood the functions of nutrition better than most others; he traces the aliment into the stomach, seems aware of the processes it has to undergo there, and hazards a conjecture that part of the chyle is taken up immediately by the pores of the cellular texture; and that the juices admitted into this membrane, served for the production of milk, the matter of which is afterterwards transported, and laid up within the breasts. He attributes to each vital part an attractive force, which it exerts on the nutritious particles, in order to incorporate and appropriate to itself those which bear to it a certain analogy. He thought that the heat generated in a living body was kept up entirely by powers of vitality; and that the external air introduced by respi-, ration, served rather to check it, by exerting a cooling effect on the pulmonary organs. He represents the human body as agitated in all its parts by an alternate flux and reflux, which carried the matters from withinoutwards, or brought them from without inwards. From this some have supposed that he understood the circulation of the blood, a supposition made two hundred years ago, and lately again brought forward by the French. physiologists. We shall not at present stay to canvass this opinion, which, however, we conceive to be founded on very unsatisfactory arguments.

After Hippocrates, the science of man was again left Plato. to the schools of philosophy, from which he had first se parated it. Plato is the first philosopher whose opinions merit peculiar notice. He wrote on the physiology of man with his accustomed elegance and splendour of diction; and he assumed the tone of an inspired prophet in decribing, with the force of enthusiasm, the grand: images that suggested themselves to his mind. Ac

cording

33

35

that of the body, which was no other than a passive or- History. gan, in which the phenomena that previously existed in the soul, in an abstract, latent manner, became evident and sensible. To the former sect belonged Democritus and Epicurus; to the latter, Zeno and Plato. The professors of the Alexandrian school, though Herophilus. they did much for the improvement of anatomy, added little to physiology. Of these Herophilus brought to some degree of perfection the doctrine of the pulse, and seems to have understood the action of the pulmonary organs more correctly than his predecessors, attributing to them a sort of natural appetite, by which they attracted and rejected the matter of respiration. He considered the nerves, the muscles, and the arteries, as the moving powers of the body.

History. Cording to him, the human body does not contain with in itself the cause of the phenomena which are the consequence or the attendants of life. It is only a passive subject, ou which the soul expresses the series of its functions, like the canvas on which the painter traces the conceptions of his inventions. He distinguishes two principles of action in man, a rational soul, on which depend reflection and intelligence, and an irrational soul, on which depend life and motion. The latter is diffused through every part of the body; and it is by means of these parts that it feels, suffers pain, or enjoys pleasure. Thus it is by means of the heart it is susceptible of courage and of passion; by the liver of desire. The head is the seat of reason; the chest, and especially the heart, the seat of strength and anger; the lungs, the general coolers of the body. One division of the irrational soul, which possesses an appetite for food, and all the necessary refreshments of the body, resides in the epigastric region; which, in the language of Plato, is a sort of stable, in which resides a voracious animal. During nutrition, the vital parts assimilate to their substance the aliments which are presented to them; and this assimilation is the consequence of an affinity that takes place between these parts and the nutritious juices. He thus seems to regard nutrition as the effect of a combat between the aliments and the parts of the animal. A young animal will receive more nourishment than one which is old, because the force of its body has more effect in overcoming the force of the nutritious substances.

34

Aristotle.

As the reciprocal action of the soul and the body on each other did not appear to him capable of being explained on the supposition of immateriality, he proposed the idea of a plastic nature, which he supposed to be an intermediate principle connecting the soul and the body.

The human body, which is entirely spongy, is exposed through every part to opposite currents of air and fire, which traverse and penetrate it, being introduced alternately by the lungs and by the skin. Hot, cold, dense, rare, and the other sensible properties of bodies, are only the causes of the phenomena which we perceive, and are, as it were, the occasions or accidents that are required to keep in play the intelligent force disseminated through nature.

Aristotle, the disciple of Plato, for a long time dis. puted with him the palm of genins and celebrity; but, as his physiological doctrines differed very little from those of his master, it is unnecessary to detail them, except to remark, that he attributed to the soul three faculties, a nutritive, a sensitive, and a rational faculty; in the first of which life is the only principle; in the second, feeling is produced; and the third is peculiar to man, and is that part of him which knows or judges. This part is either an active or a passive intellect, of which the first may be separated from the body, and is immortal; whereas the second perishes together with the body. Life, according to this philosopher, is a permanence of the soul, retained by the natural heat, the principle of which resides in the heart.

About the period which we are now considering, philosophy was divided into two sects; the materialists, who attributed the formation of all beings to the fortuitous concourse of atoms; and the spiritualists, who held that the soul enjoyed an existence anterior to 4

36

In Galen, also a disciple of this school, we find the Galen. most scientific physiologist that has yet come under our notice. He scenis first to have ascertained by experiment, that the arteries contain blood, and not air, as had been the opinion of Herophilus and his predecessors and that they possessed a moving force, independent of that which the heart exercises on the mass of blood, and he found that the contraction of the heart always alternated with a proportional dilatation. He even tried some delicate experiments, in order to ascertain the influence of the nervous system upon the sensitive and motive powers of the body, by which he found, that when a nerve was intercepted with a ligature, the part to which it led became deprived of sense and motion. He believed that the stomach, in a state of contraction, applied itself to the aliment that had been taken in; that the mesenteric veins absorbed a portion of the chyle prepared in the intestines; that the ductus choledochus carried the bile from the gall bladder into the duodenum; that the kidneys separated a part of the urine and he supposed, that another part of this fluid. passed immediately from the stomach to the bladder, through some unknown passage. He believed that the lungs transmitted to the blood contained within them, an aerial principle, destined to free them from fuliginous vapours, and to temper the excess of heat generated within the body. The obscure function of generation did not entirely escape his researches; and he made some curious attempts to find out how the sexual organs prepared the seminal fluid, and how this acted in reproduction.

For more respecting the doctrines of Galen, see the History of MEDICINE.

37

The commencement of the 13th century is the epoch The che of a material revolution in physiology. Chemistry ha-mists. ving penetrated into Europe, soon exerted its influence on most of the sciences, and especially on those connected with medicine, the doctrines of which were totally changed from their ancient simplicity, and became a farrago of the most wild and fanciful opinions. The Peripatetics and the Galenists sunk into oblivion; and the primitive qualities and occult faculties of the ancient school gave way to the fermentations and effervescences of the chemists. Albertus Magnus and Roger Bacon, when they introduced the science of chemistry, scarcely dreamed of applying it to medicine; but Arnoldus de Villa Nova undertook this application, and sought for the foundation of medical theory amid the processes of his laboratory. Paracelsus followed, and surpassed him in this chemical delirium. An enlightened chemist and a

credulous

History, credulous astrologer, his head burning with the fire of his furnaces, and his imagination filled with magical reveries, he believed himself capable of constructing a new system of philosophy, from examining the course of the stars, and the products of his alembics. With the daring assurance of inspiration, he declared man to be composed of sulphur, mercury, and salt; and, having traced the origin of all diseases to certain chemical operations, he flattered himself, that by means of his arcana he could preserve health, and prolong, to an indefinite extent, the natural duration of human exist

38 Van Helmont.

39

Des Cartes.

40

Discovery

of the circulation,

ence.

Van Helmont, the disciple of Paracelsus, not less fanciful, but more scientific than his master, saw the necessity of something more than chemical principles to explain the functions of the animal machine. He therefore introduced his archæus, an intelligent being who established his throne in the epigastric region, having several subaltern ministers under him, who presided over the several functions of the body, and whose chief seats were, the head, the chest, and the belly.

In the philosophy of Des Cartes, the separate existence of the vital principle is entirely rejected. He availed himself of the progress that had been made by Willis, and some other anatomists, in the investigation of the nervous system, to form an hypothesis of the vital functions, founded on the supposition of the nervous floid, or what was then called the animal spirits; and this nervous fluid was assumed independently of the sensitive soul, to explain the appearances of sensation and voluntary motion.

The discovery of the valves in the veins by Fabricius; of the lymphatics by Rudbeck and Bartholin; of the lacteals by Asellius, and of the circulation of the blood by Harvey, all of which took place during the 17th century, gave to physiology an interest and a clearness which it never possessed before that period. Some account of the discoveries in the circulating and absorbing systems, hath been already given under ANATOMY; but as these discoveries have been productive of great advantages, both in general physiology, and in medicine, it will be worth while briefly to trace their origin and progress.

To begin with the circulation of the blood. Hippocrates speaks of the usual and constant motion of the blood, of the veins and arteries as the fountains of human nature, as the rivers that water the whole body, and which if they be dried up man dies. He says, that the blood-vessels are, for this reason, everywhere dispersed through the whole body; that they give spirits, moisture, and motion; that they all spring from one; and that this one has no beginning and no end, for where there is a circle there is no beginning. In such language was the prince of physicians accustomed to express his vague ideas of a circulation; for so far was he from having acquired accurate conceptions on this subject, that when he saw the motions of the heart, he believed that the auricles were two bellows to draw in air, and to ventilate the blood.

When after his time anatomy came to be more studied, the notions of the ancients respecting the blood were better defined; and, however chimerical they may seem to us, they were partly derived from dissection and experiment. On opening dead bodies, they found that the arteries were almost empty, and that very nearly the

whole of the blood was collected in the veins, and in History, the right auricle and ventricle of the heart. They therefore concluded that the right ventricle was a sort of laboratory; that it attracted the blood from the cavæ; by some operation rendered it fit for the purpose of nutrition, and then returned it by the way that it came. From the almost empty state of the arteries, they were led to suppose that the right ventricle prepared air, and that this air was conveyed by the arteries to temper the heat of the several parts to which the branches of the veins were distributed.

To this last notion, entertained by Erasistratus, Galen added an important discovery. By certain experiments, he proved, that the arteries contained blood as well as the veins. But this discovery was the occasion of some embarrassment. How was the blood to get from the right to the left ventricle? To solve the difficulty in which this new discovery had involved him, he supposed that the branches of the veins and arteries anastomosed; that when the blood was carried to the lungs by the pulmonary vein, it was partly prevented by the valves from returning; that therefore during the contraction of the thorax it passed through the small inosculating branches to the pulmozary vein, and was thence conveyed along with the air to the left ventricle to flow in the aorta. This opinion, so agreeable to fact, unfor tunately afterwards gave place to another that was the result of mere speculation. This notion was that the left ventricle received air by the pulmonary vein; and that all its blood was derived through pores in the sep tum of the heart.

The passage through the septum being once suggest: ed, and happening to be more easily conceived than one through the lungs, it was generally supposed the only one for a number of centuries; and supported likewise, as it was thought, by Galen's authority, it was deemed blasphemy in the schools of medicine to talk of another. In 1543, however, Vesalius having published his immortal work upon the structure of the human body, and given his reasons in the sixth book why he ventured to dissent from Galen, he particularly shewed how it was impossible that the blood could pass through the septum of the heart. His reasoning roused the attention of anatomists; and every one grew eager to discover the real passage which the blood must take in going from the right to the left ventricle. The discovery of this was first made by Michael Servede, a Spanish physician, who published his opinion in 1553. He expressly says, that the blood does not pass through the septum of the heart, as is commonly believed, but is conveyed by an admirable contrivance from the right ventricle of the heart, by a long passage through the lungs. This opinion was deemed heretic, and Servede's book was suppressed by public authority. Soon after, however, the same discovery was made by Realdus Columbus, an Italian professor, who published his account in 1559. It farther appears, that Andreas Casalpinus, who published in 1571, and again in 1593, was acquainted, not only with the lesser circulation, but observed, that the blood sometimes flowed from the branches of the veins towards their trunks; and that when a vein was tied with a ligature, it swelled between the ligature and the distant extremity of the vein, and not between the ligature and the heart. He thence inferred, that the veins and arteries opened into each other, and ventured to as

sert

We learn from Galen, that certain vessels had been History.

History. sert that the blood could not return by the arteries to the left ventricle of the heart. He did not, however, discover the true circulation, but most unaccountably maintained with Aristotle, of whom he was a zealous disciple, that the blood flowed like the tides of Euripus, backward and forward in the same channel, and supposed that it flowed from the arteries into the veins during sleep, and back again from the veins into the arteries till the waking state.

In 1574, Hieronymus Fabricius ab Aquapendente, while he was seeking for a cause by which to explain the various swellings of some veins, which had arisen from friction and ligature, he, to his great joy and astonishment, discovered their valves in one of his dissections; and here again the true theory of circulation seemed almost unavoidable. Yet whoever reads the small treatise De Venarum Ostiolis, first printed by Fabricius in 1603, will soon perceive that he was as far from entertaining a just notion of the circulation as his predecessors. Notwithstanding all that he saw, he still was of opinion that the blood flowed from the heart to the extremities even in the veins. He thought that the valves were intended by nature only to check and moderate its force. He calls them an instance of admirable wisdom, and mistakes his own awkward conjecture for one of the designs of infinite intelligence. In another respect, it must be confessed that he bore no inconsiderable share in promoting the discovery of the circulation. By writing on the valves, the formation of the foetus, and chick in ovo, he directed the attention of his pupil Harvey to those subjects where it was likely that the motion of the blood would frequently occur.

Harvey, who was born in 1578, and graduated at Padua in 1602, examined the valves with more accuracy than Fabricius, and explained their use in a treatise which he published soon after his graduation. In 1616, he taught the true doctrine of the circulation in his lectures, and published on the subject in 1628. He was the first author who spoke consistently of the motion of the blood, and who, unbiassed by the doctrine of the ancients, drew rational conclusions from his experiments and observations. His books present us with many indications of a great mind, acute discernment, unwearied application, original remark, bold inquiry, and a clear, forcible, and manly reasoning; and every one who considers the surprise which his doctrine occasioned among the anatomists of those days, the strong opposition that it met with from some, and the numerous and powerful prejudices which it had to encounter from the sanction of time and of great names, must allow it was new; and that the author has from its importance, a title to rank in the first class of eminent discoverers, ancient or modern.

We have been the more particular in tracing the progressive discovery of the circulation, and in attempting to shew that the real merit of the discovery is due to Harvey, because both the abilities and the originality of this eminent man have been called in question by his countryman Dr William Hunter; and many hints are given in the writings of some foreign physiologists, that the circulation was at least guessed at by Hippocrates

and other ancient writers.

We shall be less minute in tracing the discovery of the lymphatic system, because this has been related more at large in the article ANATOMY,

3

Discovery

seen in kids by Erasistratus, which appear to have been 41 lacteals, though he called them arteries. These lacteals were, however, first accurately distinguished in 1622, of the by Asellius, who printed his account in 1627. In sorbent sy 1651, Pecquet published his account of the thoracic stem. duct, which appears, however, to have been seen before by Eustachius. In 1653, Bartholin published on the lymphatics, which had been some time before discovered by Rudbeck. In 1654, Glisson ascribed to these vessels the office of carrying back the lubricating lymph from the arteries into the blood, or considered them as absorbents. In 1664, the valves of these vessels were discovered by Swammerdam, and a year after, an account of them was given by Ruysch. The farther discoveries of Nuck, Nouges, Warton, Steno, Hunter, Monro, Hewson, Cruikshank, Sheldon, Mascagni, &c. have nearly completed our knowledge of the absorbent system, and its uses.

In the latter end of the 17th century, some important discoveries were made on the subject of respiration, by our countryman Mayow; and these were supported by the observations of Lower, Verheyen, and Borelli. These discoveries, however, lay dormant till they were brought into recollection a hundred years after in consequence of the experiments of Priestley and Lavoisier.

During the 17th century, considerable progress was made in completing the knowledge of the internal organs of generation. Much was done in this way by De Graaff and Malpighi, and Leuwenhoeck, the two latter of whom made several discoveries with the assistance of their microscopes, though Leuwenhoeck founded on his observations a theory of generation which at this day appears not a little ridiculous.

42

The beginning of the 18th century is remarkable for System of the promulgation of a new physiological doctrine, found- Boerhaave. ed on a mistaken application of the circulation of the blood. We allude to the system of Boerhaave. This great physician supposed that all the functions of the living body, excepting the will, are carried on by mechanical movements, susceptible of rigid calculation, which necessarily succeed each other in the organs, from the time that life commences. These movements are brought into action as soon as the animal begins to respire, and are the consequence of an impulsive power in the heart, renewed by means of the influence of the nervous fluid i brought from the brain. He conceived the living body to be merely a hydraulic machine, in which the heart performs the office of a piston, and that the alternate contractions and dilatations that take place without intermission in that organ, are owing to the alternately increased and diminished compression of the nerves that are distributed to the heart. When a contraction takes place, the blood fills the large arteries, and thus distends and compresses them; when the principal nerves of the heart, which pass between these arteries, must of course become compressed, and thus their influence being di minished, a relaxation takes place. But in proportion as the heart is relaxed, the large arteries become empty, and consequently cease to compress the nerves, which thus recovering their influence, reanimate the heart to a new contraction. Thus succeed each other without

interruption the movements which form the mechanical principle of all the sensible motions that we observe in the animal machine.

Proceeding

History.

43.

Mechanists.

44

System of

Proceeding on these principles, Boerhaave conceived some very strange notions respecting the constituent properties of the living fluids, in which he saw no other mark of vitality than the globular form of their particles. He confined all the functions of the several organs to the operation of rounding into spheres the particles of the fluids which are presented to them, or of preserving that form in those which they already contained. He thought that the lungs were chiefly of advantage, because they contained within them a complete series of vessels, in which the particles of the blood can receive all those dimensions which may fit them to circulate through the rest of the body. The greater or less velocity with which the fluids circulate through the secretory organs, constitutes the principal difference in the

nature of the secretions. Various orders of vessels receive the blood and other fluids which pass through these divisions, subject to the laws of hydraulics; and when a fluid got by chance into an order of vessels that was not fitted to receive it, some disease was the consequence. Every thing in the animal machine was reduced to an assemblage of conduits, canals, cords, levers, pulleys, and other mechanical contrivances, put into action by mechanical means.

Thus was completed the system of mechanical physiology, which was begun some time before by Bellini and Borelli; and this system maintained its ground in defiance of observation and common sense, till about the middle of the 18th century. In the mean time, however, there arose two men, whose enlarged ideas and acute genius induced them to dissent from the received opinions of the day, and to think for themselves. These were Hoffman and Stahl, who, though they did not, any more than Boerhaave, form complete or unobjectionable theories, contributed much to improve our ideas of the animal economy.

Hoffman saw, that in the living body we ought not Hoffman. to separate the principle of vitality from the general properties of matter. He believed that that principle, susceptible in itself of activity and motion, was sufficient for all the occasions and all the functions of the body which it animated. The animal body was not, in his eyes, an hydraulical elastic machine, formed of solids and canals, differing only in size, form, elasticity and force. He saw, that if the solids act upon the fluids, thesc must, in their turn, react upon the solids; and that life could subsist only by these mutual actions and reactions. The essential cause of life, according to Hoffman, is the progressive motion of the blood, occasioned by the impulse of the heart, and kept up by the alternate contractions and dilatations of the vessels. These contractions and dilatations are the consequence of the force of an elasticity inherent in the vascular fibres, and this force is still farther promoted by the different structure of these elastic fibres, which is such that they can be penetrated by the blood and the nervous fluid. This last fluid he imagined to be composed of aerial and etherial particles enveloped in a certain portion of a very pure subtile lymph, that served them as a vehicle. By this fluid the cavities of the nerves are filled, and it constitutes the sensitive soul, in which resides the seat of the passions. Now, all the functions. even those which we attribute to the sentient principle, are the effect of physical powers, whose mechanism has, however, something more sublime and more exalted VOL. XVI, Part II.

with respect to the animal operations than to others. History. If all the nervous, vascular, and membranous parts, preserve a moderate degree of action, and a moderate state of tension and relaxation, the solids are subjected to oscillatory motions which balance each other, and produce a proper equilibrium in the system. In this state, all the operations of the body and the mind take place with proper regularity; and this happy harmony, by assuring to the animal the entire plenitude of its existence, becomes the foundation of health. This degree of moderate tension is always more or less altered in a state of disease.

45

Little satisfied with all the theories founded on a gross of Stahl. mechanism, and convinced of their insufficiency to explain he phenomena of vitality, Stuhl admitted forces that were something more than mechanical, and that were directed by an intelligent principle which applies them to their destined uses, and which, by distributing them with a wise economy, poportions or accommodates them to the different occasions of the individual. His disciples consider Stahl as the first modern writer who has treated the science of man on a general plan, and according to a philosophical arrangement; and as his doctrine has still numerous advocates in the medical schools of France, we shall be somewhat more particular on it than on that of Hoffman.

In determining the limits between medicine and the other physical sciences, Stahl commences with separating from the former all those principles which, though true in themselves, have no relation to the nature of that science, which he considers as originating in observation alone. The knowledge of the physical state of the animal body cannot, he thinks, throw any light, either on the injuries to which it is exposed, or on the means of preventing or removing them. Consequently it is of little use in medicine, and has no right to govern an art, the object of which is, to remedy those injuries that threaten the human body. He proves that living bodies are freed from the necessary laws of mechanics, because all their actions tend to one common end;-an end which embraces the whole chain of the movements essential to life, and the means established for its preservation. The human body, by means of this mixture of mechanical and vital powers, tends naturally to self-destruction; but, on the other hand, the organic structure to which is attached the exercise of the actions peculiar to the human species, is founded on this mixture. It is therefore necessary that the body should be in a state of resisting this tendency, in order that it may be sustained; and as the corruptibility inherent in its nature, pursues it through every period of its existence, the opposing action necessary to prevent the corruption from taking place, must also be exercised without intermission. It is this preserving action that constitutes the essence of life.

The preservation of the body is indeed effected by a sort of mechanical action; it requires the corporeal organs as its instruments, and it depends on different coexistent and successive actions. Health is the result of that just conformation of the organs which enables them to perform their functions with ease.

The exact conformity which subsists between the structure of each organ, and the functions it is destined to perform, demonstrates to the philosophical eye an intelligent and wise principle, that in the formation of organized bodies directs and prescribes every thing in 3 M the