Part 3 (1/2)
FOOTNOTES: [1] By cachexia is understood a condition of malnutrition and emaciation which is usually accompanied by a pale sallow color of the skin.
[2] By trauma is understood a wound or injury of any sort.
CHAPTER IV
THE REACTIONS OF THE TISSUES OF THE BODY TO INJURIES.--INFLAMMATION.-- THE CHANGES IN THE BLOOD IN THIS.--THE EMIGRATION OF THE CORPUSCLES OF THE BLOOD.--THE EVIDENT CHANGES IN THE INJURED PART AND THE MANNER IN WHICH THESE ARE PRODUCED.--HEAT, REDNESS, SWELLING AND PAIN.--THE PRODUCTION OF BLISTERS BY SUNBURN.--THE CHANGES IN THE CELLS OF AN INJURED PART.--THE CELLS WHICH MIGRATE FROM THE BLOOD-VESSELS ACT AS PHAGOCYTES.--THE MACROPHAGES.--THE MICROPHAGES.--CHEMOTROPISM.--THE HEALING OF INFLAMMATION.--THE REMOVAL OF THE CAUSE.--CELL REPAIR AND NEW FORMATION.--NEW FORMATION OF BLOOD-VESSELS.--ACUTE AND CHRONIC INFLAMMATION.--THE APPARENTLY PURPOSEFUL CHARACTER OF THE CHANGES IN INFLAMMATION.
Injury and repair have already been briefly considered in their relation to the normal body and to old age; there are, however, certain phenomena included under the term inflammation which follow the more extensive injuries and demand a closer consideration than was given in Chapter II. These phenomena differ in degree and character; they are affected by the nature of the injurious agent and the intensity of its action, by the character of the tissue which is affected and by variations in individual resistance to injury. A blow which would have no effect upon the general surface of the body may produce serious results if it fall upon the eye, and less serious results for a robust than for a weak individual.
Most of the changes which take place after an injury and their sequence can be followed under the microscope. If the thin membrane between the toes of a living frog be placed under the microscope the blood vessels and the circulating blood can be distinctly seen in the thin tissue between the transparent surfaces. The arteries, the capillaries and veins can be distinguished, the arteries by the changing rapidity of the blood stream within them, there being a quickening of the flow corresponding with each contraction of the heart; the veins appear as large vessels in which the blood flows regularly (Fig. 11). Between the veins and arteries is a large number of capillaries with thin transparent walls and a diameter no greater than that of the single blood corpuscles; they receive the blood from the arteries and the flow in them is continuous. The white and red blood corpuscles can be distinguished, the red appearing as oval discs and the white as colorless spheres. In the arteries and veins the red corpuscles remain in the centre of the vessels appearing as a rapidly moving red core, and between this core and the wall of the vessels is a layer of clear fluid in which the white corpuscles move more slowly, often turning over and over as a ball rolls along the table.
If, now, the web be injured by p.r.i.c.king it or placing some irritating substance upon it, a change takes place in the circulation. The arteries and the veins become dilated and the flow of blood more rapid, so rapid, indeed, that it is difficult to distinguish the single corpuscles. In a short while the rapidity of flow in the dilated vessels diminishes, becoming slower than the normal, and the separation between the red and white corpuscles is not so evident. In the slowly moving stream the white corpuscles move much more slowly than do the red, and hence acc.u.mulate in the vessels lining the inner surface and later become attached to this and cease to move forward.
The attached corpuscles then begin to move as does an amoeba, sending out projections, some one of which penetrates the wall, and following this the corpuscles creep through. Red corpuscles also pa.s.s out of the vessels, this taking place in the capillaries; the white corpuscles, on the other hand, pa.s.s through the small veins. Not only do the white corpuscles pa.s.s through the vessels, but the blood fluid also pa.s.ses out. The corpuscles which have pa.s.sed into the tissue around the vessels are carried away by the outstreaming fluid, and the web becomes swollen from the increased amount of fluid which it contains.
The injured area of the web is more sensitive than a corresponding uninjured area and the foot is more quickly moved if it be touched. If the injury has been very slight, observation of the area on the following day will show no change beyond a slight dilatation of the vessels and a great acc.u.mulation of cells in the tissue.
Everyone has experienced the effect of such changes as have been described in this simple experiment. An inflamed part on the surface of the body is redder than the normal, swollen, hot and painful. The usual red tinge of the skin is due to the red blood contained in the vessels, and the color is intensified when, owing to the dilatation, the vessels contain more blood. The inflamed area feels hot, and if the temperature be taken it may be two or three degrees warmer than a corresponding area. The increased heat is due to the richer circulation. Heat is produced in the interior of the body chiefly in the muscles and great glands, and the increased afflux of blood brings more heat to the surface. A certain degree of swelling of the tissue is due to the dilatation of the vessels; but this is a negligible factor as compared with the effect of the presence of the fluid and cells of the exudate.[1] The fluid distends the tissue s.p.a.ces, and it may pa.s.s from the tissue and acc.u.mulate on surfaces or in the large cavities within the body. The greatly increased discharge from the nose in a ”cold in the head” is due to the exudation formed in the acutely inflamed tissue, and which readily pa.s.ses through the thin epithelial covering. Various degrees of inflammation of the skin may be produced by the action of the sun, the injury being due not to the heat but to the actinic rays. In a mild degree of exposure only redness and a strong sense of heat are produced, but in prolonged exposure an exudate is formed which causes the skin to swell and blisters to form, these being due to the exudate which pa.s.ses through the lower layers of the cells of the epidermis and collects beneath the impervious upper layer, detaching this from its connections. If a small wad of cotton, soaked in strong ammonia, be placed on the skin and covered with a thimble and removed after two minutes, minute blisters of exudate slowly form at the spot.
The pain in an inflamed part is due to a number of factors, but chiefly to the increased pressure upon the sensory nerves caused by the exudate. The pain varies so greatly in degree and character that parts which ordinarily have little sensation may become exquisitely painful when inflamed. The pain is usually greater when the affected part is dense and unyielding, as the membranes around bones and teeth.
The pain is often intermittent, there being acute paroxysms synchronous with the pulse, this being due to momentary increase of pressure when more blood is forced into the part at each contraction of the heart. The pain may also be due to the direct action of an injurious substance upon the sensory nerves, as in the case of the sting of an insect where the pain is immediate and most intense before the exudate has begun to appear.
When an inflamed area is examined, after twenty-four hours, by hardening the tissue in some of the fluids used for this purpose and cutting it into very thin slices by means of an instrument called a microtome, the microscope shows a series of changes which were not apparent on naked eye examination. The texture is looser, due to the exudate which has dilated all the s.p.a.ces in the tissue. Red and white corpuscles in varying numbers and proportions infiltrate the tissue; all the cells which belong to the part, even those forming the walls of the vessels, are swollen, the nuclei contain more chromatin, and the changes in the nuclei which indicate that the cells are multiplying appear. The blood vessels are dilated, and the part in every way gives the indication of a more active life within it. There are also evidences of the tissue injury which has called forth all the changes which we have considered. (Fig. 15.)
[Ill.u.s.tration: FIG. 15--A SECTION OF AN INFLAMED LUNG SHOWING THE EXUDATE WITHIN THE AIR s.p.a.cES. Compare this with Fig 6. Fig 15 is from the human lung, in which the air s.p.a.ces are much larger than in the mouse.]
The microscopic examination of any normal tissue of the body shows within it a variable number of cells which have no intimate a.s.sociation with the structure of the part and do not seem to partic.i.p.ate in its function. They are found in situations which indicate that these cells have power of active independent motion. In the inflamed tissue a greatly increased number of these cells is found, but they do not appear until the height of the process has pa.s.sed, usually not before thirty-six or forty-eight hours after the injury has been received. The numbers present depend much upon the character of the agent which has produced the injury, and they may be more numerous than the ordinary leucocytes which migrate from the blood vessels.
All these changes which an injured part undergoes are found when closely a.n.a.lyzed to be purposeful; that is, they are in accord with the conditions under which the living matter acts, and they seem to facilitate the operation of these conditions. It has been said that the life of the organism depends upon the coordinated activity of the living units or cells of which it is composed. The cells receive from the blood material for the purpose of function, for cell repair and renewal, and the products of waste must be removed. In the injury which has been produced in the tissue all the cells have suffered, some possibly displaced from their connections, others may have been completely destroyed, others have sustained varying degrees of injury.
If the injury be of an infectious character, that is, produced by bacteria, these may be present in the part and continue to exert injury by the poisonous substances which they produce, or if the injury has been produced by the action of some other sort of poison, this may be present in concentrated form, or the injury may have been the result of the presence of a foreign body in the part. Under these conditions, since the usual activities of the cells in the injured part will not suffice to restore the integrity of the tissue, repair and cell formation must be more active than usual, any injurious substances must be removed or such changes must take place in the tissue that the cell life adapts itself to new conditions.
[Ill.u.s.tration: FIG. 16.--PHAGOCYTOSIS. _a_, _b_, _c_ are the microphages or the bacterial phagocytes. (_a_) Contains a number of round bacteria, and (_b_) similar bacteria arranged in chains, and (_c_) a number of rod-shaped bacteria (_d_) Is a cell phagocyte or macrophage which contains five red blood corpuscles.]
All life in the tissues depends upon the circulation of the blood.
There is definite relation between the activity of cells and the blood supply; a part, for instance, which is in active function receives a greater supply of blood by means of dilatation of the arteries which supply it. If the body be exactly balanced longitudinally on a platform, reading or any exercise of the brain causes the head end to sink owing to the relatively greater amount of blood which the brain receives when in active function. The regulation of the blood supply is effected by means of nerves which act upon the muscular walls of the arteries causing, by the contraction or the relaxation of the muscle, diminution or dilatation of the calibre of the vessel. After injury the dilatation of the vessels with the greater afflux of blood to the part is the effect of the greatly increased cell activity, and is a necessity for this. In many forms of disease it has been found that by increasing the blood flow to a part and producing an active circulation in it, that recovery more readily takes place and many of the procedures which have been found useful in inflammation, such as hot applications, act by increasing the blood flow. So intimate is the a.s.sociation between cell activity, as shown in repair and new formation of cells, and the blood flow, that new blood vessels frequently develop by means of which the capacity for nutrition is still more increased. The cornea or transparent part of the eye contains no blood vessels, the cells which it contains being nourished by the tissue fluid which comes from the outside and circulates in small communicating s.p.a.ces. If the centre of the cornea be injured, the cells of the blood vessels in the tissue around the cornea multiply and form new vessels which grow into the cornea and appear as a pink fringe around the periphery; when repair has taken place the newly formed vessels disappear.
The exudate from the blood vessels in various ways a.s.sists in repair.
An injurious substance in the tissue may be so diluted by the fluid that its action is minimized. A small crystal of salt is irritating to the eye, but a much greater amount of the same substance in dilute solution causes no irritation. The poisonous substances produced by bacteria are diluted and washed away from the part by the exudate. Not only is there a greater amount of tissue fluid in the inflamed part, but the circulation of this is also increased, as is shown by comparing the outflow in the lymphatic vessels with the normal. The fluid exudate which has come from the blood and differs but slightly from the blood fluid exerts not only the purely physical action of removing and diluting injurious substances, but in many cases has a remarkable power, exercised particularly on bacterial poisons, of neutralizing poisons or so changing their character that they cease to be injurious.
We have learned, chiefly from the work of Metschnikoff, that those white corpuscles or leucocytes which migrate from the vessels in the greatest numbers have marked phagocytic properties, that is, they can devour other living things and thus destroy them just as do the amoebae. In inflammations produced by bacteria there is a very active migration of these cells from the vessels; they acc.u.mulate in the tissue and devour the bacteria. They may be present in such ma.s.ses as to form a dense wall around the bacteria, thus acting as a physical bar to their further extension. The other form of amoeboid cell, which Metschnikoff calls the macrophage, has more feeble phagocytic action towards bacteria, and these are rarely found enclosed within them. It is chiefly by means of their activity that other sorts of substances are removed. They often contain dead cells or cell fragments, and when haemorrhage takes place in a tissue they enclose and remove the granules of blood pigment which result. They often join together, forming connected ma.s.ses, and surround such a foreign body as a hair, or a thread which the surgeon places in a wound to close it. They may destroy living cells, and do this seemingly when certain cells are in too great numbers and superfluous in a part, their action tending to restore the cell equilibrium. The foreign cells do even more than this: they themselves may be devoured by the growing cells of the tissue, seemingly being actuated by the same supreme idea of sacrifice which led Buddha to give himself to the tigress.
The explanation of most of the changes which take place in inflammation is obvious. It is a definite property of all living things that repair takes place after injury, and certain of the changes are only an accentuation of those which take place in the usual life; but others, such as the formation of the exudate, are unusual; not only is the outpouring of fluid greatly increased, but its character is changed. In the normal transudation[2] the substances on which the coagulation of the blood depends pa.s.s through the vessel wall to a very slight extent, but the exudate may contain the coagulable material in such amounts that it easily clots. The interchange between the fluid outside the vessels and the blood fluid takes place by means of filtration and osmosis. There is a greater pressure in the vessels than in the fluid outside of them, and the fluid filters through the wall as fluid filters through a thin membrane outside of the body. Osmosis takes place when two fluids of different osmotic pressure are separated by animal membrane.
Difference in osmotic pressure is due to differences in molecular concentration, the greater the number of molecules the greater is the pressure, and the greater rapidity of flow is from the fluid of less pressure to the fluid of greater pressure. The molecular concentration of tissue and blood fluid is constantly being equalized by the process of osmosis. In the injured tissue the conditions are more favorable for the fluid of the blood to pa.s.s from the vessels: by filtration, because owing to the dilatation of the arteries there is increased amount of blood and greater pressure within the vessels, and the filtering membrane is also thinner because the same amount of membrane (here the wall of the vessel) must cover the larger surface produced by the dilatation. It is, moreover, very generally believed that there are minute openings in the walls of the capillaries, and these would become larger in the dilated vessel just as openings in a sheet of rubber become larger when this is stretched. Osmosis towards the tissue is favored because, owing to destructive processes the molecular pressure in the injured area is increased; an injured tissue has been shown to take up fluid more readily outside of the body than a corresponding uninjured tissue. The slowing of the blood stream, in spite of the dilatation of the vessels, is due to the greater friction of the suspended corpuscles on the walls of the vessels. This is due to the loss from the blood of the outstreaming fluid and the relative increase in the number of corpuscles, added to by the unevenness of surface which the attached corpuscles produce.
The wonderful migration of the leucocytes, which seems to show a conscious protective action on their part, takes place under the action of conditions which influence the movement of cells. When an actively moving amoeba is observed it is seen that the motion is not the result of chance, for it is influenced by conditions external to the organism; certain substances are found to attract the amoebae towards them and other substances to repel them. These influences or forces affecting the movements of organisms are known as _tropisms_, and play a large part in nature; the attraction of various organisms towards a source of light is known as _heliotropism_, and there are many other instances of such attraction. The leucocytes as free moving cells also come under the influence of such tropisms. When a small capillary tube having one end sealed is partially filled with the bacteria which produce abscess and placed beneath the skin it quickly becomes filled with leucocytes, these being attracted by the bacteria it contains. Dead cells exert a similar attraction for the large phagocytes. Such attraction is called _chemotropism_ and is supposed to be due in the cases mentioned, to the action of chemical substances such as are given off by the bacteria or the dead cells. The direction of motion is due to stimulation of that part of the body of the leucocyte which is towards the source of the stimulus. The presence in the injured part of bacteria or of injured and dead cells exerts an attraction for the leucocytes within the vessels causing their migration. When the centre of the cornea is injured, this tissue having no vessels, all the vascular phenomena take place in the white part of the eye immediately around the cornea, this becoming red and congested. The migration of leucocytes from the vessels takes place chiefly on the side towards the cornea, and the migrated cells make their way along the devious tracts of the communicating lymph s.p.a.ces to the area of injury. The objection may be raised that it is difficult to think of a chemical substance produced in an injured area no larger than a millimeter, diffusing through the cornea and reaching the vessels outside this in such quant.i.ty and concentration as to affect their contents, nor has there been any evidence presented that definite chemical substances are produced in injured tissues; but there is no difficulty in view of the possibilities. It is not necessary to a.s.sume that an actual substance so diffuses itself, but the influence exerted may be thought of as a force, possibly some form of molecular motion, which is set in action at the area of injury and extends from this. No actual substance pa.s.ses along a nerve when it conveys an impulse.
We have left the injured area with an increased amount of fluid and cells within it, with the blood vessels dilated and with both cells and fluid streaming through their walls, and the cells belonging to the area actively repairing damages and multiplying. The process will continue as long as the cause which produces the injury continues to act, and will gradually cease with the discontinuance of this action, and this may be brought about in various ways. A foreign body may be mechanically removed, as when a thorn is plucked out; or bacteria may be destroyed by the leucocytes; or a poison, such as the sting of an insect, may be diluted by the exudate until it be no longer injurious, or it may be neutralized. Even without the removal of the cause the power of adaptation will enable the life of the affected part to go on, less perfectly perhaps, in the new environment. The excess of fluid is removed by the outflow exceeding the inflow, or it may pa.s.s to some one of the surfaces of the body, or in other cases an incision favors its escape. The excess of cells is in part removed with the fluid, in part they disappear by undergoing solution and in part they are devoured by other cells. With the diminis.h.i.+ng cell activity the blood vessels resume their usual calibre, and when the newly formed vessels become redundant they disappear by undergoing atrophy in the same way as other tissues which have become useless.