Chapter VII: Section III (1)

_Of Disease in general.--The nature of the Plague as a Disease
considered.--Of Contagion.--Whether the Plague is really Contagious
or not.--Medical History of the Distemper.--Inquiry into its
Immediate Causes, and whether an approaching Plague is indicated by
any visible Signs._

Hitherto we have considered the origin of the plague entirely in a moral point of view. We have seen, that, in conformity to the general opinion of mankind, it may reasonably be supposed to have been inflicted upon mankind, the Jews particularly, for their transgressions; that, having been once introduced, it has been perpetuated, and spread from nation to nation, and that in proportion to the degree of immorality of a certain kind prevailing through the world. From this it is naturally to be inferred, that, were the human race to live at peace with one another, to disperse themselves over the face of the earth for the purpose of improving it by cultivation, and were they to be contented with what the produce of each country affords, there would be no plague among them. But we know that such a reformation is not to be expected, and we must take the world as we find it. The question then is, By what means shall individuals secure themselves from being destroyed by a plague which shall happen to invade any country; or how shall a person, already infected with it, be restored to health? For this purpose let us begin with considering the nature of disease in general, and of the plague particularly.

As to disease in general, physicians have differed very considerably in their definitions; and, though many have been given, few seem to be unexceptionable. That of Dr. Fordyce seems to be among the clearest and most expressive. “Disease (says he) is such an alteration in the chemical properties of the fluids or solids, or of their organization, or of the action of the moving powers, as produces an inability or difficulty of performing the functions of the whole or any part of the system, or pain, or preternatural evacuation.” But as this definition, however just, cannot be easily understood by such as are unaccustomed to medical language, I shall attempt the following explanation of the animal economy, and the diseases to which it is subject.

1. By nature our bodies are formed of certain solid and fluid parts, operating upon one another in a manner of which we know but little. Anatomists have described the structure of the human body and its parts in a certain degree, but have always found themselves lost in an inconceivable minuteness of texture. The whole structure of the human body, visible and invisible, is called its ORGANIZATION.

2. This organized body is acted upon by certain powers residing in the atmosphere, by which it becomes endowed with LIFE.

3. The operation of those powers upon a well organized body constitutes that agreeable and vigorous state which we call HEALTH.

4. The operation of any other power, substituted in place of the natural one, even upon a body perfectly organized, produces a state very different from health; commonly attended with some uneasy sensations, and which is called DISEASE. I say it is _commonly_ attended with uneasiness, but not always; for many persons within a few hours, nay, a few minutes, of their death, have imagined themselves quite recovered and well. To illustrate the meaning of what is said of the substitution of any power instead of the natural one: It is natural for man to breathe air of a certain quality; and while he does so he continues in health; but let him breathe the vapour of burning charcoal, or of fermenting liquor, mixed in considerable quantity with the air to which he has been accustomed, and he will very soon find himself diseased. Many other kinds of elastic fluids may be substituted instead of the vapour just mentioned, all of which will in a short time produce a disease in the most healthy man. The state of a diseased body being very different from that of a sound one, the appearances are consequently very different. The various appearances of disease in the human body are called _symptoms_ of that disease, from a Greek word signifying _appearance_.[44]

[44] “_Symptom_ (says Dr. Fordyce) is the Greek name for
_appearance_:” but, from the strict etymology of the word, it
ought rather to be translated _accident_. The universal consent of
physicians, however, has applied it to every appearance produced in
the human body by any distemper whatever.

5. A disease proves mortal only by the DISORGANIZATION of the body. By disorganization I mean any considerable alteration in the structure of the body, visible or invisible. The truth of this will appear from a consideration of the method by which animals may be recovered, after being to appearance dead by breathing the vapour of charcoal, or fixed air in any other form, viz. by plunging them in cold water. In a cave in Italy a continual stream of this kind of air issues from the ground. It rises but a small way, so that a man may safely enter, because his head is above the vapour; but, if he brings a small dog with him, the animal, in consequence of breathing the pernicious fluid, falls down as if dead, and would very soon die if left there. By throwing it into a lake in the neighbourhood, (cold water of any kind would answer as well) it recovers. In the dissection of some unfortunate people, who have been killed by breathing this pernicious fume, a manifest disorganization has been observed, viz. a rarefaction of the blood, and too great dilation, or even rupture, of the small vessels.

6. A disease cannot always be cured by removing the cause which brought it on: it is necessary also to repair the injury done to the organization. This is exemplified in the case of the dog just mentioned. Taking him out of the vapour is not sufficient for his recovery, because the organization of the body is injured; the cold water by contracting the vessels repairs the injury, and the cure is completed. To the entire preservation of this organization it is probably owing, that people have frequently recovered after being thought dead for a long time.[45]

[45] Dr. Anthony Fothergill, in his prize dissertation upon the
suspension of vital action, quotes some experiments of Dr. Kite,
in which he was able to restore to life animals that had been
immersed in water for eight, ten or twelve minutes, though he
acknowledges that this operation, though performed with great
attention, often failed; while other animals, that had been longer
immersed, recovered spontaneously. He further adds, that if it be
not attempted before the convulsions of the animal cease, which
on an average of many experiments happens in about eleven minutes
and a half, it will not be sufficient to renew the vital motions.
But, “among the human species (says Dr. Fothergill) there are not
wanting well authenticated instances of spontaneous recovery at
an incomparably longer interval, and after every external mark of
life had disappeared. Such is the latent energy of the heart, that
it sometimes, after remaining several hours quiescent, renews on
a sudden the secret springs of life, surmounts the barriers of
the resisting blood, and restores circulation with all the other
functions. Hence the unexpected recoveries from death-like syncope
brought on by sudden terror, or great effusions of blood, even after
the funeral obsequies have been prepared. Hence some persons have
accidentally been brought to life, even after interment, by the rude
motion produced in sacrilegious attempts to wrest rings or bracelets
from the apparently dead body.”

Several surprising instances of the recovery of persons supposed to
be dead, even of the plague, are given by Fabricius Hildanus; to
one of which Dr. Fothergill seems to allude in the above quotation.
Hildanus relates, that in the year 1357, when the plague raged
violently at Cologne, a certain noble lady, by name _Reichmuth
Adoleh_, being seized with the disease, was thought to have died,
and was buried accordingly. Her husband, out of affection, would not
take off her wedding ring, which she happened to have on her finger.
The undertakers being acquainted with this circumstance, next night
came to the church where she was buried, opened the sepulchre, and
prepared to take off the ring; when to their utter astonishment she
began to raise herself up in the coffin. Struck with consternation
they fled in the utmost haste, leaving to the fortunate lady the
lantern with which they lighted themselves to the church, and by
means of which she now found out where she was, and after being
come to herself, returned to her own house. Here being known by her
voice, and the ring she wore, she found admittance, and by means of
a generous diet gradually regained her health; bringing her husband
afterwards three children, and surviving the accident many years.

A second instance no less remarkable is of a woman of the name of
_Nicolle Lentille_, who, being supposed dead of the plague, had been
thrown into a pit with a great number of the bodies of others, dead
of the same distemper. After lying there a whole night, she came to
herself in the morning, but neither knew at first where she was, nor,
when she did, could she find any means of escaping, or extricating
herself from the heap of dead bodies with which she was oppressed.
Being at a distance from any house, her cries were of no avail, and,
in the mean time, having taken no nourishment for four days, she was
so tormented with hunger that she eat part of the cloth which covered
her face. At last, after remaining twenty-four hours in this dreadful
situation, the pit being opened to bury some other person, she
exerted her utmost endeavours in calling for assistance, and at last
was heard by those who stood round. Being taken up and brought home,
she presently recovered, and lived several years after.

A third example is given by our author of one who, being carried to
a church to be buried, had his face previously sprinkled with holy
water by a priest. But this was no sooner done than he shuddered and
opened his eyes in a fright; on which he was carried home, recovered,
and lived eight years after. Other examples might be brought, but
these are sufficient to show what dreadful accidents may ensue from
early burials, and how cautious people ought to be in consigning
their friends and relations to the dust from whence they were taken.

7. When the organization of the body is injured, the action of the natural powers themselves occasions uneasiness, and increases the disease. The cure then is, to substitute instead of the natural power, as far as possible, the action of some other power till the organization is restored; after which the natural power must be again allowed to act, or a disease of another kind will take place. This may be exemplified in a consumption of the lungs; where, that part being very much disorganized, pure air renders the disease worse; and the sick are relieved by mixing with the common atmosphere such kinds of air or vapour as would prove pernicious to people in health. But, supposing this method to be successful, and the consumption to be entirely removed, it is plain that the use of the pure atmosphere must be resumed, or the impure air would bring on a disease in the same manner as on a healthy person.

8. The body is wasted in the natural operations of life; part of it passing off with the vapour of the breath, part by insensible perspiration, &c. Hence it naturally tends to disorganization and death, unless the waste be repaired.

9. This natural waste of the body is repaired, and health kept up, by the food and drink taken into the stomach.

10. Hence arises another set of diseases; for as the reparation of the waste, just mentioned, depends on the proper action of the stomach upon the food, and the assimilation of the latter with the substance of the body, it is plain that this operation depends both on the proper quality of the food, and the sound state of the stomach itself.

11. The body is composed of solids and fluids of different kinds, every one of which is subject to diseases peculiar to itself; but, by reason of the connexion of the parts of the body with one another, it is impossible that any one can be very much disordered without affecting all the rest. As the bond of connexion, however, is in many cases totally invisible to us, surprising instances frequently occur of one part being affected in consequence of an injury done to another very distant from it. This connexion between all parts of the body is called SYMPATHY. Dr. Gardiner of Edinburgh, in his observations on the animal economy, &c. says, that “the stomach is the principal seat of many of the most remarkable sympathetic affections which happen in valetudinary states of the body. Every disorder accompanied with severe pain affects the stomach, whilst this viscus affects not only in its diseased state every part of the system, but at other times the effects of healthful stimuli applied to it are instantly communicated to the rest of the body, as when we take food, wine, or medicine.” Dr. Darwin in his Zoonomia informs us that the stomach is said to sympathize with almost every part of the body; but Dr. Moore, in his medical sketches, tells us that the _heart_ possesses a greater share of sympathy than any other part in the body, and next to it the stomach.[46]

[46] Dr. Gardiner, in his observations above quoted, gives the
following curious anecdote. “An unmarried lady, of a healthy
constitution, has such a peculiarity in the structure of her nerves,
that, though she can, in general, bear strong odours as well as
most people, yet she cannot suffer a rose to be in her bosom, or
to hold it in her hand a few minutes, without becoming faint, and
having an inclination to vomit. Conserve of roses, rose-water, and
similar articles made from roses, have more powerful effects upon
her, and usually excite vomiting. Going into a room where any of her
companions are washing with rose-water, never fails to produce this
effect; nor does she recover of her indisposition in less than two
hours.”

12. The solid parts of the body are the bones, the muscles, brain and nerves; the fluids are, the blood, and others produced from it. The bones are known to every one; the muscles are the fleshy parts throughout the whole body; and the nerves are a kind of cords seemingly originating from the brain, and from thence accompanying the blood vessels through all parts of the body.

13. Much has been disputed about what is to be accounted the primary part of the body, on which all the rest depend; and one class of disputants have arranged themselves on the side of the _blood_, and the other on that of the _nerves_. The dispute is like one about the beginning of a circle. It cannot be decided, because the blood cannot act without nerves, nor the nerves without blood. I speak of the human body, being aware that in some animals the position may be controverted. The following is a concise state of the matter.

14. All the blood in the body passes through the heart; which has four cavities; two called _ventricles_, and two _auricles_. These, from their position in the body, are called the _right_ and _left_. The right ventricle communicates with the right auricle, as does also the left ventricle with the left auricle; but there is no communication between the right ventricle and the left, nor between the right auricle and the left. Through these cavities all the blood passes to every part of the body, and returns from every part; but, as in the former case, we are here at a great loss where to begin its motion; for this is precisely to find the beginning of a circle. As we must begin somewhere, however, we shall do so with the right ventricle of the heart. This receives the blood returning from all parts of the body, and propels it into the right ventricle; not the whole quantity at once, for it cannot contain one half of it; but by degrees. The auricle contracts as soon as it is full; and in the time that the auricle fills, the ventricle contracts, so that it may be empty, and ready to receive the blood from the auricle. By the contraction of the right ventricle the blood is driven into the pulmonary artery, and passes into the lungs. Here the artery branches into an infinite number of small vessels much finer than hairs; and these again, uniting into larger trunks, form at last the pulmonary vein, which brings back the blood to the heart. The pulmonary vein is inserted into the left auricle of the heart, which, as soon as it is filled with blood, contracts, and expels the blood from it into the left ventricle. From the left ventricle issues a large artery called the _aorta_, which by its branches supplies the whole body with the vital fluid. In all parts of the body the arteries divide themselves into innumerable small branches, which terminate in veins equally small as in the lungs; but it has been disputed whether the arteries and veins actually join each other in the form of vessels, or whether the arteries deposit the blood in small cells, from which the veins suck it up. The dispute is of no consequence, nor can it be absolutely decided, on account of the exceeding smallness of the vessels; though the microscopical observations are rather favourable to the opinion of a continuation of vessels. The veins from all parts of the body unite into larger vessels, and these again uniting with one another, form at last one very large vein called the vena cava, which opens into the right auricle of the heart, from which the circulation goes on as already described. The two ventricles of the heart, and all the veins throughout the body, are furnished with a kind of valves, which allow the blood to proceed in the way of circulation, but prevent its returning in a contrary direction.

15. The lungs, through which all the blood in the body passes, receive likewise the air which we draw in every time we breathe. They consist of two large bodies called _lobes_; from their situation called the right and left. The air is conveyed into them by the wind-pipe, called also the _trachea_, and the _aspera arteria_. On entering the cavity of the breast, the wind-pipe divides into two large branches called the _bronchiæ_; one of which goes to the right and the other to the left lobe of the lungs. By the further division and subdivision of these vessels the lungs are filled with an innumerable multitude of little tubes, terminating in exceedingly minute bladders or cells, which are the final receptacles of the air sucked in when we breathe. Each of these cells is surrounded with a kind of network of blood-vessels exceedingly small, and consisting of very thin membranes; so that, in passing through the lungs, the blood is exposed as much as possible to the action of the air.

16. It is a matter of great importance to find out what is the use of this exposure of the blood; and great disputes have taken place concerning it. In former times it was supposed that the blood received from the air a _vital spirit_, without which it would have been totally incapable of performing its offices in the body. Later physiologists endeavoured to explode this notion. Dr. Hales particularly, by shewing that the circulation of the blood through the lungs might be continued by inflating and contracting them alternately by the fumes of burning brimstone, endeavoured to prove that the use of the air was only to give the lungs an opportunity of dilating and contracting alternately, by which means principally he thought the circulation might be carried on. This continued to be the most common hypothesis as late as the time of Dr. Huxham. It was however thought also that by the compression of the air the blood was altered in its texture, its bulk, &c. Accordingly Dr. Huxham tells us in the preface to his treatise on air and epidemic diseases, that “air fit for respiration ought neither to be too hot, nor very cold; for the use of the inspired air is to temperate the blood, which would otherwise grow too hot, and putrefy, as is evident from the experiment of the most excellent Boerhaave made in a hot house; for, if the air is more hot, or even equally hot, as the blood of any animal, it certainly soon dies.”[47]

[47] This certainly does not hold good if we suppose the heat of the
atmosphere to be indicated by a thermometer; for we are assured that
animals can live in a heat much superior to that which raises the
mercury to 97.

17. The modern discoveries in the composition of air, have tended greatly to elucidate the use of this fluid in the lungs, and its action on the blood in respiration. Dr. Priestly first determined it to be what he terms a _phlogistic process_, i. e. a process by which the parts of the blood no longer proper to be retained among the rest, or at least some of them, are carried off. That _something_ is carried off either from the lungs themselves, or from the blood circulating through them, is evident; for the air which is taken into the lungs in a dry state, comes out of them extremely moist, and loaded with vapour. An essential change is also made in the nature of the air itself; for it now assumes in a great measure the nature of what has been called fixed air, or the fume of charcoal, or fermenting liquor, and thus becomes unfit for being breathed a second time. This change is made by the addition of some terrestrial substance to the pure atmosphere, which the latter volatilizes and carries along with it.[48]

[48] The discoveries of modern chemists have determined that the
aerial fluid, termed _fixed air_ or _carbonic acid_, and which is
nearly the same with the vapour arising from fermenting liquor, and
is also largely contained in the fume of burning charcoal, is not
a simple but a compound substance; one part consisting of the pure
part of the atmosphere, or _oxygene_, the other of real charcoal. The
proportions, according to M. Chaptal, are 12,0288 parts of charcoal
to 56,687 of oxygen.

18. But, whatever may be _carried off_ from the blood, during its passage through the lungs, something is certainly _added_ to it, for the blood in the pulmonary artery is of a dark red, but when it has undergone the action of the air in the lungs, and returns by the pulmonary vein, it is then of a bright scarlet, which colour it retains through all the arteries of the body, but loses it on its return through the veins. This scarlet colour is communicated to blood in all cases when exposed to the air; and Dr. Priestley has observed that it is acted upon by the air even through a bladder; much more then must it be so through those very thin membranes which form the coats of the fine pulmonary vessels. What this subtile matter is which the blood receives, shall be afterwards inquired into; at present it is sufficient to take notice that it is absolutely necessary, for the purposes of life, that the blood should pass through the lungs: for, as Dr. Huxham observes, “we see neither nutrition, nor the motion of the muscles, performed by any blood that hath not passed through the lungs; this is observable from the coronary arteries[49] to the ultimate ramifications of the aorta.” As the previous circulation of the blood through the lungs therefore is absolutely necessary to the growth and life of the body, and as the blood certainly receives _something_ from the air, we must account this a proof, and no inconsiderable one, that the air contains a _vital spirit_, which it imparts to the blood in the lungs. But, before we proceed farther on this subject, it is proper to take some notice of

[49] The name of the vessels by which the heart itself is supplied
with blood. These come from the aorta by the circuitous way of the
lungs.

19. _The nerves._ These, which constitute such a remarkable and important part of the human body, are white cords, of a soft pulpy substance, defended by a tough skin which goes along with them as far as they can be traced. All the nerves either originate from the brain, or terminate in it. The former doctrine hath been generally adopted, and in conformity to that doctrine the following account of the nerves is laid down. The brain is enclosed in the cavity of the scull, but not without the intervention of two membranes, called the _dura_ and _pia mater_, to prevent injury from the hard bones, as well as for other purposes. The brain is divided into two lobes, the right and left. It is composed of two different kinds of substance, the outermost called the cortical, the innermost the medullary substance; the latter seems composed of fine fibres. The whole of the medullary part of the brain terminates in a substance called the _cerebellum_, very much resembling the brain, but smaller. The cerebellum terminates in another substance resembling the medullary part of the brain, called the _medulla oblongata_. The cerebellum lies in the back part of the head, and the medulla oblongata under it. The latter terminates in the spinal marrow, extending from the lower and back part of the head to the lower extremity of the back bone, and is enclosed in the hollow of that bone. The nerves proceed from these four substances, viz. the brain, the cerebellum, the medulla oblongata, and spinal marrow. As they pass to all parts of the body they accompany the arteries, dividing with them into innumerable small branches; but they do not return with the veins; so that they seem not to contain any fluid which goes and comes, or which circulates like the blood. The nervous fluid, if any such there be, seems to move constantly one way, either to the brain or from it.

20. Hitherto we have noticed only things which are evident to our senses, and which the industry of anatomists has abundantly evinced; but now our subject renders it necessary to step aside a little into the obscure regions of theory and conjecture. The muscles, as we have formerly said, are the fleshy parts of the body; and by them all the motions of the body are performed. The flesh is distributed into distinct portions, each of which is enclosed in a membrane belonging to itself. Each of these portions is a muscle, and each muscle has a branch of an artery and the branch of a nerve belonging to it. On both these the action of the muscle depends; for, if we cut the nerve belonging to a muscle, it immediately loses all power of action; and if we cut the artery which accompanies the nerve, it does the same. As therefore the blood is found to receive _something_ from the air, and as it loses this when passing through the arteries, and as the nerves lose their power when the communication with the blood is cut off, it seems extremely probable, that what is imbibed by the blood in the lungs is taken up by the fine ramifications of the nerves, and is no other than the immediate principle of life and sensation. Thus we will establish a doctrine directly opposite to that commonly received; for, instead of supposing that the nerves originate from the brain, we are now led to suppose that they terminate in it. Instead of supposing that the sensations originate in the brain, we will be led to suppose that every sensation originates in the organ appointed for that sensation. Thus we are conscious that our _eyes_, not our _brain_, are the parts of our body which immediately perceive the light; our fingers, or any other parts of the body, _feel_ what is applied to them; and of consequence we have reason to believe that the _animal spirits_, _nervous fluid_, or whatever we please to call it, proceed from the surface of the body inwards to the brain, not outwards from the brain to the surface of the body. The brain itself seems to resemble a large collection or reservoir of water, in which the sensations, like so many small streams from every part of the body, unite, and in which our intellectual faculties reside in a manner totally inexplicable by us. Thus far it seemed necessary to theorise, in order to form some idea, however obscured, of the connexion between the nerves and our sensitive and intellectual, or, if we please to call them so, our _spiritual_ faculties.

21. In consequence of this very intimate connexion between the blood and nerves, it is easy to see that any injury done to the one may very greatly affect the other; and that a very slight, nay, to us imperceptible, change in the organization of either, may produce the most grievous, and even incurable disorders throughout the whole body, or in any particular organ. Let us now consider a little farther the blood-vessels.

22. It hath been a question, whether in the structure of these vessels nature hath observed an exact proportion. For instance, if the blood passes by a kind of starts through four cavities, as we are assured that it does, it seems natural to suppose that these four should be exactly equal. This, however, hath been denied; and some, from its accommodating the human frame to their theory, have fancied that they saw the use of such disproportionate work. Dr. Huxham expresses himself in the following words: “Nor doth the air only refrigerate the blood, but, by preventing its too great ebullition, and condensing it, hinders it from bursting the vessels. This indeed is of exceeding great importance, if, with the very learned _Helvetius_, we suppose the capacity of the right ventricle of the heart to be greater than that of the left, and that the pulmonary arteries are larger than the correspondent veins; for it thence follows, that the blood ought to be considerably condensed by the inspired air, that an equal quantity of blood may be received, in one and the same time, by the pulmonary veins and left ventricle of the heart, that is thrown off from the right ventricle, and through the more capacious pulmonary arteries. This indeed many deny, asserting quite the contrary. It is necessary, however, that the aorta should receive as much blood from the left ventricle of the heart, as is thrown off from the right ventricle through the pulmonary artery; and that in the very same and equal time, or a fatal deluge would soon overwhelm the lungs, because the contraction of each ventricle is made at one and the same time; we always find therefore the aorta and pulmonary artery, in a natural state, equal on this account; also the capacity of the ventricles ought to be equal, that they may receive, in one and the same space of time, equal quantities of blood,” &c.

If any thing farther is necessary upon this subject, we may still observe, that if the blood were at all condensed by the air, it would be so unequally, because the air is at some times much colder than at others; and thus the disproportion of the cavities of the heart to one another could not fail of producing the most disagreeable if not fatal effects. We often see what terrible consequences ensue upon the enlargement of any part of an artery near the heart; and these would, sometimes at least, be felt by every individual.[50]

[50] But there is a still more egregious blunder, and this the more
surprising as it has been very general among physiologists, viz.
that when an artery branches into two the capacity of the branches
taken together is greater than that of the trunk. This would make the
whole arterial system one continued _aneurism_,[51] and, instead of
promoting the circulation of the blood, would in the most effectual
manner prevent it. In what manner an error so extraordinary in its
nature could pass the mathematical physicians of the last century,
I cannot imagine; but certain it is, that, in the year 1780 or
1781, the Edinburgh College were schooled on this subject by one of
their own students named _John Theodore Vander Kemp_, a Dutchman.
This gentleman found, by accurate mensuration, that when an artery
divides, if the diameters of the two branches are made the two
shorter sides of a right-angled triangle, the diameter of the trunk
will be the hypothenuse; and thus, as the areas of circles are to one
another in proportion to the squares of their diameters, the sum of
the areas of the two branches will be equal to the area of the trunk.
On looking into Blumenbach’s physiology, I find the same remark.

[51] An aneurism is a preternatural enlargement of an artery. The
blood stagnates in that place, and at length eats through the flesh
and skin.

It is true, indeed, that this objection will in some degree hold, even though we suppose all the cavities of the heart to be equal, and the capacities of the blood vessels to be perfectly uniform throughout the whole body. For, if we suppose the blood to be at all condensed in the lungs by the coldness of the atmosphere, it must undoubtedly follow, that while passing from them it occupies less space than before it arrives at them. Hence the pulmonary vein, the left auricle of the heart, the left ventricle, the aorta, and all the rest of the arteries for a considerable way, must be comparatively empty, even though they receive as much fluid as fills the vena cava, right auricle and ventricle of the heart, and pulmonary artery. The equality which ought to prevail in the system, and which indeed cannot be dispensed with, can only take place in those remoter branches of the arteries in which the blood has reassumed its former state of dilation or rarefaction.

23. If we consider this matter attentively, we shall find it not a little mysterious. Every time we breathe out the air we have sucked into our lungs, a considerable quantity of moist vapour is breathed out along with it; but it has been proved by undeniable experiments that the emission of aqueous vapour from any substance cools it in proportion to the quantity of vapour emitted. Every breath we draw, then, cools the lungs, and consequently the blood, to a certain degree, and, as the number of times that we breathe in a day is exceedingly great, the cold produced by the evaporation ought to be in proportion. But we see that, notwithstanding all this cooling, whether we breathe cold air or hot air, the temperature of the body remains still the same. The air then, though constantly carrying off the heat of the body, does not cool it in the least by its action on the lungs. The only possible way of solving this apparent contradiction is, by supposing that the air, when acting upon the blood in the lungs, leaves precisely as much heat as it carries off, and therefore, though we breathe ever so long, we cannot by this means become either hotter or colder.

24. To illustrate this subject, we might now enter into an inquiry concerning the origin and cause of animal heat; but this will be touched upon hereafter. We shall here only take notice that the heat of the body is almost universally allowed to proceed from the lungs. It has likewise been demonstrated, that the air does in fact contain an incredible quantity of heat, even when it appears to us to be extremely cold. A certain proportion of this heat is separated from it every time we breathe; and if, either by the mixture of other fluids with the air we breathe, or by any change in the organization of the body itself, a greater or smaller proportion of heat should be communicated to the blood, disease must ensue.

25. To sum up then what has been said concerning the blood and nerves: The whole mass of fluid passes from the right side of the heart to the lungs. In the lungs it receives from the air something[52] necessary to the functions of life and sensation, and purifies itself from those matters which might prove pernicious. From the lungs it passes to the left side of the heart, and thence through the whole body. In its passage through the body, it is accompanied with nerves, which, taking up from the arterial blood that _vital spirit_ received from the air, convey it to all the organs of motion, of sensation, and to the brain, where the whole powers of perception being united form our _intellectual faculties_, and, as far as our senses can perceive, the human spirit itself. The blood, thus deprived of its spirit, is collected from all parts of the body by the veins, and returned to the right side of the heart, from whence it is again sent to the lungs, and the process carried on as before. This hypothesis concerning the peculiar function of the nerves I first inserted in the Encyclopædia Britannica, second edition, under the article BLOOD, in the year 1778. It has been since continued in the third Scots edition, and from thence into the Irish and American editions.

[52] It seems now to be proved beyond a doubt that this
_something_ so long unknown is that fluid called by Dr. Priestley
_dephlogisticated air_, and by Lavoisier _oxygen_.

26. It has already been observed, that the body is subjected to a continual waste. One source of this waste is the breath, by which a considerable part passes off in vapour. A great quantity also passes off by the pores of the skin; frequently in a perceptible liquid called _sweat_, but oftener in an invisible vapour from all parts of the body, called _insensible perspiration_. The latter has been thought to be the great source of waste to the human body; and it is certain, that if any person in health be weighed when he rises in the morning, he will be found considerably lighter than when he went to bed. The loss of weight in this case proceeds not only from the pores of the skin, but from the lungs; but though physicians have made a general allowance for both these, I have not heard of any experiment by which we can determine how much passes off by the one, and how much by the other, nor indeed does it appear easy to make such an experiment. Galen plainly overlooks the perspiration from the lungs entirely. “This excrementitious vapour (says he) is expelled through small orifices, which the Greeks call pores, dispersed all over the body, and especially over the skin, partly by sweat, and partly by insensible perspiration, which escapes the sight, and is known to few.” Sanctorius, and the succeeding writers, have classed both together indiscriminately; allowing the discharge to be so great, that if eight pounds of aliment be taken in, five of them pass off in this manner. In a system of anatomy, published at Edinburgh in 1791, the author says, that the discharge by the skin “is even much larger than this (the discharge from the lungs we may suppose) since it not only throws off a quantity of the aliment, but likewise what is added to the blood by inhalation, which, entering often in a very considerable quantity, is thus again expelled.” The same author likewise says, that the “perspirable matter from the skin is principally water,” and that it issues in such quantity as to be seen in subterraneous caverns evidently flying off from the surface of the body like a dense vapour. But other physiologists, particularly Dr. Blumenbach, inform us, that the matter of insensible perspiration is quite similar to the discharge from the lungs, particularly containing a great quantity of fixed air. The same account is given in Chaptal’s chemistry, on the authority of Messrs. Milly and Fouquet. This may be looked upon as a valuable discovery, especially in conjunction with that related by Drs. Beddoes and Girtanner, viz. that the flesh of animals contains a quantity of oxygen. Dr. Girtanner obtained a quantity of this air from the raw flesh of animals, and says that it may be repeatedly obtained by exposing the flesh to the atmosphere, and distilling with a heat of 60 or 70 degrees of Reaumur’s thermometer (something below that of boiling water.) Hence it is natural to conclude, that, as the discharge from the lungs purifies the blood from its useless parts, so does the insensible discharge from the skin purify the solid parts from those particles which are no longer useful. The probability of this also becomes greater by considering, that in diseases, when the quantity of matter to be thrown off is very great, the skin becomes foul, the teeth furred with black sordes, &c. all which disappear as soon as the quantity of the offensive particles is reduced to its natural standard. As to any considerable quantity of aqueous vapour being discharged this way, unless in case of sweat, it does not seem probable; for in such a case our clothes would always be moist; and in the night time the accumulation of moisture would certainly be perceptible. The sweat is entirely of a different nature from the insensible perspiration, and blood and even _sand_ has been known to issue through the skin along with it. (_See the Anatomical System above quoted._)

27. This very considerable waste of the body is repaired by the aliment taken into the stomach. In the mouth it is mixed with a considerable quantity of the liquid called _saliva_, and in the stomach with another called the _gastric juice_, with which that organ always abounds. From the stomach it passes into the intestines, where it is mixed with other two fluids; one called the _pancreatic juice_, the other the _bile_. This last is of a yellow colour, and is sometimes produced in enormous quantities, insomuch that Dr. Wade, in his account of the fevers in Bengal, mentions some patients who have voided by stool half a gallon of bilious matter in one day.

28. In the stomach principally the aliment undergoes a certain change called _digestion_, by which it becomes capable of being converted into the substance of the body. Much has been inquired and disputed, to no purpose, about the nature of this change, and how it is effected. One party has declared for _attrition_; a second for _putrefaction_; a third for _heat_; a fourth have supposed that our meat was digested by _chewing_; as if, like the lobster, people had teeth in their stomach! and, lastly, some learned moderns, after much pains and trouble, have found out that it is digested by _solution_. Dr. Moore has summed up the discoveries concerning digestion in the following words: “The food, being previously divided and blended with the saliva and air by mastication, (chewing) is swallowed, and meets in the stomach with the gastric juice, whose dissolving power, assisted by the natural heat of the place, is the principal agent in digestion. The process is completed by the pancreatic juice and bile, the nutritious parts of the food being by this process converted into chyle for the support of the body, and the grosser parts thrown out.”[53]

[53] Moore’s Medical Sketches.

29. The inside of the stomach and intestines are full of the mouths of innumerable small vessels, which continually suck up from the aliment, as it passes downwards, the finer parts, in form of a white liquid, called _chyle_; and from the whiteness of their colour the vessels have the name of _lacteals_, from the Latin word _lac_, milk. After passing through the substance of the stomach and intestines, and running along the membrane called the _mesentery_, to which the intestines are attached, the lacteals unite in a large reservoir called the _thoracic duct_; and this again opens into a large vein on the left side, called the _subclavian_, which conveys the blood from half the upper part of the body; soon after terminating in the vena cava, by which the chyle is conveyed to the heart, thence to the lungs, and so on in the common course of circulation. The conversion of the chyle into blood is called the process of _sanguification_.

30. The blood, thus formed out of the aliment we swallow, is not one uniform fluid like water, but composed of three distinct substances; one, which gives it the red colour, and seems to be composed of little round globules; another, quite colourless, but of a viscid nature, and which very soon coagulates, called the _lymph_; and a third, of a yellowish colour, and retaining its fluidity much longer, called the _serum_. A remarkable property of this last fluid is, that air can act through it upon the blood; for Dr. Priestly found that a portion of black blood assumed a bright, florid colour from the air, even though covered with serum an inch deep. When blood is drawn, the red globules are detained by the lymph which coagulates, and both together form the red mass called _crassamentum_; the serum remaining fluid, and retaining its name.

31. Besides these fluids, the blood either invisibly contains, or is capable of being converted into, a great many others; for all the fluids in the body are separated from it, and all of them, the bile only excepted, from the arterial blood, before it has lost that portion of its spirit which it imbibes from the air. When a fluid is to be secreted, sometimes it is done only by an infinity of small vessels branching off from the arteries, and depositing the liquids which pass through them in particular places; and such are the fluids which moisten the inside of the body, and which are carried off by the breath, or by sweat. But this separation does not by any means hinder the artery from terminating in its usual way in a vein, for in no case is the whole substance of the blood converted into any other liquid; all of them appear to be contained in it. But the greatest number of fluids are separated by means of certain substances called _glands_. These are small round or oval shaped bodies; each of them enclosed in a membrane or skin which separates it from the other parts, and each furnished with a small tube called the _excretory duct_, through which the liquor separated in the gland passes to its place of destination. Each gland has also an artery and nerve, and a vein to bring back the blood after it has parted with the fluid intended to be separated. The bile is separated in the liver from the blood of a large vein called the _vena portarum_, formed by the union of some of the veins of the intestines and mesentery. This vein branches out through the liver like an artery, terminating in other veins, which at last bring back the blood to the heart.

32. As the human body is thus furnished with an apparatus for separating and carrying off, it is also furnished with one for absorbing or taking in. All the inward parts of the body are moist; and the moisture is furnished by the small vessels above described, and which separate part of the lymph from the blood. By such continual separation the cavities of the belly, breast, brain, &c. would soon be filled with liquid, were not some means provided for carrying it off as fast as it is formed. The means in question are a set of small vessels called _lymphatics_. These “arise from the internal surface of the breast, belly, and every cavity of the body; they also overspread the whole external surface of the body, and large lymphatic vessels are usually found close to the large blood vessels of the extremities, besides those small superficial ones which lie above the muscles in the cellular membrane (the fat or rather the membrane containing it.) The large viscera generally have two sets of lymphatics, one lying on the surface of the viscus, and the other accompanying the blood vessels belonging to it. The faculty of absorption, though refused to the lymphatics, was ascribed by many anatomists to common veins, and this opinion continued to prevail in some degree, until Hunter and Monro totally overturned it, exploding at the same time the notion that any of the lymphatics are continuations of arteries, and establishing, beyond a doubt, that all are absorbent vessels.”[54] All the lymphatics terminate in the thoracic duct; so that the liquid separated by the _exhalant arteries_ (so the vessels are termed by which that fluid is separated) is again mixed with the blood, and again performs the same offices.

[54] Moore’s Medical Sketches.