Part 2

I. General Division of the Capillaries.

Since this system is not destined merely to unite the arteries to the veins, and change the black blood to red, it is evident that other fluids besides the blood, must circulate in them; this is in fact what observation proves. There are many parts in the animal economy, in which white fluids alone circulate. We know the hypothetical opinions of Boerhaave upon the white arteries, the decreasing vessels, &c. We shall find these opinions in all books; I shall only say here what accurate observation shows us. That there is in the general capillary system, parts in which the blood especially moves, others pervaded only by white, or greyish fluids, &c. is a thing of inspection, and has no need of proofs. But what is the proportion of these fluids in the different organs? it is this that must be examined; now there are parts where the blood predominates almost exclusively in the capillary system, others where it exists in part, and in which there is a portion of different fluids, others in fine in which these fluids alone are found.

Of the Organs in which the Capillaries contain only Blood.

It appears that in the muscular system, in the spleen, in certain parts of the mucous surfaces, as in the pituitary membrane, &c. the blood so predominates in the capillary tubes, that almost every other fluid is unknown in them; thus fine injections demonstrate few other vessels; the arteries and the veins are seen there in great abundance. The blood, or its colouring matter, is in them, as I shall say, in two different states; in one, it stagnates and serves then to colour the organ; in the other, it circulates and contributes to its nutrition, its excitement, &c.

Of the Organs in which the Capillaries contain Blood and Fluids differing from it.

These organs are the most numerous in the animal economy. The bones, the cellular texture, the serous membranes, a part of the fibrous system, the skin, the vascular parietes, the glands, &c. &c. exhibit this arrangement in a very remarkable manner.

To give an idea of the capillary system in this kind of organs, let us take one in which it is easy to examine it, the serous membranes, for example. When we lay them bare in a living animal, their transparency permits us to see in an evident manner that they contain but very little blood in their capillary system; there are many branches under them, but they appear to be only contiguous; raise, for example, in a small living guinea-pig the peritoneal coat of the stomach; the red arteries, which at first view appeared to be in this coat, remain untouched. These membranes, certainly owe their white or greyish colour to the small quantity of blood they receive from their small vessels, to which the next trunks give rise. After having thus exposed a serous membrane, in order to see the quantity of blood that is found in it in a natural state, irritate it with any stimulant; at the end of some time, it will be covered by an infinity of reddish streaks, which will be so numerous, that they will change its whiteness into the red colour of the mucous surfaces.

Force fine injections into a dead body, they will so fill the capillary system of the serous surfaces, of those of the peritoneum, for example, that those surfaces will be wholly black, and appear formed only by a net-work of vessels, whilst very few are apparent in the living body, because it is not the blood that fills them. When we cannot open animals to convince us of this, surgical operations, in which the intestines are laid bare, the peritoneum being untouched, wounds of the abdomen, the cesarian operation, &c. will prove incontestably that in the natural state, the blood fills ten and even twenty times less of the vessels, upon the serous surfaces, than injections show us in their texture.

Examine these surfaces in chronic and acute inflammations, of which they are the seat, in the first especially, they exhibit a vascular net-work, so full of blood, that their redness is often deeper than that of the muscles.

All the organs of which I have spoken above, exhibit the same phenomenon. Observe what takes place in the skin; fine injections show there infinitely more vessels than are filled with blood in a natural state; the face of a child well injected, is wholly black. Who does not know that oftentimes from the effect of passion, the blood fills with great rapidity, in the skin of the cheeks, numerous vessels, which in a calm state of mind are not apparent?

Examine the conjunctiva, taken as an example in inflammations; frequently in a short time it changes its white to a bright red, because the blood fills vessels, in which it did not before pass; you can easily distinguish these vessels with the naked eye; you can see that the blood accumulated in this membrane, is not effused, but that it is contained in real vessels.

I take for example the organs that have one of their surfaces free from adhesion, because the state of the capillary system is more easily distinguished in them; but the others present the same phenomenon; we shall see that the cellular texture, certain fibrous organs, &c. &c. examined comparatively, on the one hand in animals that we dissect alive, on the other in an inflammatory state or after fine injections, present a much less number of vessels in the first, than the second case.

It can then be established as an incontestable fact, that in many of the organs of the animal economy, the general capillary system is, in the ordinary state, entered in part by blood, in part by other different fluids, that appear to be white.

The proportions vary singularly; thus the capillary system of the serous membranes contains hardly any blood as I have said; that of the skin more; the mucous surfaces still more, &c. But whatever may be the relation, the difference is not less real in the capillary system.

Perhaps also there are always in this system empty vessels, destined to receive fluids under certain circumstances; thus the urethra, the excretory ducts in certain cases, the orifices of the lacteals in the intervals of digestion, contain nothing. It is difficult to conceive of the rapidity of the passage of the blood in the capillaries of the face, and in those of other parts of the skin, if these vessels contain a fluid, which is to be displaced by the blood. However, nothing but what is founded upon experiment can serve to decide this question.

Of the Organs in which the Capillaries do not contain Blood.

These organs are less numerous than the preceding. They are the tendons, the cartilages, the hair, certain ligaments, &c. Dissected in a living animal, not a single drop of blood escapes from these organs, and yet there is no doubt that capillaries exist in them; oftentimes very fine injections demonstrate them there. Inflammation, also, frequently fills these capillaries with blood. Into the hair, this fluid enters in the plica polonica, &c. The non-vascular appearance of these organs in the living body, is illusory; it is because their fluids are divided into very small streams, the circulation of them is more slow, and their colour different from the blood, that we cannot perceive them.

II. Difference of Organs in respect to the number of their Capillaries.

Though the capillaries exist every where, yet they are more or less numerous in the different organs; in making fine injections, it is easy to be convinced of this. What anatomist has not been struck by the prodigious number of vessels in this way developed, upon the skin, the serous surfaces, the cellular texture, &c. compared with those in the fibrous organs, in the muscles even, &c.?

I have sought for the cause of this difference, and it has not appeared difficult to find it, since where injections develop few capillaries, there is only nutrition going on, as the bones, the muscles, the cartilages, the fibrous bodies, &c. are a constant proof; on the contrary, in all those in which many fluids enter, there are, besides nutrition, other functions, such as exhalation and secretion. Hence why a serous surface, almost as white as a cartilage in the living body, becomes ten times darker than it by the same fine injection; why the skin, compared to the fibrous organs, exhibits the same phenomenon; why in proportion to the arteries that enter a muscle and a gland, the latter admits much more injection than the other.

These observations, which are uniform and invariable, prove that the capillary system is as much more developed in a part, as it has more functions to sustain. Observe in fine, that it is a kind of depot in which the fluids remain for a certain time, before serving for nutrition, exhalation and secretion. Where these three functions are united, it is necessary that there should be more fluids there, than where only one of them exists; hence more capillary vessels.

The capillary system is not then in the organs in proportion to their size; a portion of the pleura contains more vessels than a tendon that is ten times larger. It is the nutritive substance that fills the place that these vessels do not occupy.

We might, from what has just been said, divide the systems into two classes, from the development of their capillaries; place on one side the serous, the mucous, the glandular, the dermoid, the synovial, the cellular, &c.; on the other the osseous, the cartilaginous, the fibrous, the arterial, the venous, the fibro-cartilaginous, &c. The first class surpasses the second, considerably, in the number of its small vessels. Observe, also, that inflammation, different eruptions, all the affections, in which there is, as it is called, an afflux of humours to a part, are infinitely more frequent in the first than the second class, because all these affections are essentially seated in the capillary system, which is more developed in them.

Asphyxia, apoplexy, and all the affections that make the black blood stagnate in the general capillary system, prove the same thing; in fact, examine the livid head of one who has died of asphyxia, or of apoplexy, you will see that it is especially in the skin and the cellular texture that the blood is arrested; that the muscles, the aponeuroses, exhibit besides the blood ordinarily found in them, only a small quantity of superabundant fluid, in comparison with what there is in the first organs.

Remarks upon Injections.

From all that has been said, it is evident, that fine injections, which form a convenient method of knowing the capillary system of an organ, cannot show which vessels of this system admit red blood, and which circulate only white fluids. In fact, the injected matter passes equally into each, and we cannot distinguish that, which in the living body is very distinct.

In order to form a precise and accurate idea of the quantity of blood that enters each of the organic systems during life, it is indispensably necessary to dissect those systems during life. I shall frequently have occasion in this work, to make this truth felt, which appears to me to be of much importance in many respects. If a fine injection but partially succeeds, it almost always shows vessels that really exist, but which were not sanguineous during life. Even the coarse injections of our dissecting rooms frequently exhibit these phenomena, especially in the face, the neck, &c.; and much more so, if the matter injected is very delicate, and managed with much address. I cannot conceive why physiologists have always taken as an indication of the blood vessels, the state of injected organs; by opening any part of a living animal, they may clearly see how deceptive this method is.

Injections are of no advantage except in the great vessels, in which the blood circulates in a mass by the influence of the heart; in the capillaries, they do not reach the precise point that exists in nature.