TWO GREAT MUCOUS MEMBRANES.

53

uniformly exists in which fat never appears, while there are some situations-for example, in the cancelli of bones-exhibiting a copious deposit of fat, without any vestige of areolar tissue. And as the two tissues seem to be quite distinct, even in those situations where both exist in proximity, the old term adipose cellular tissue should be discarded.

Fat is not to be confounded with adipose tissue. The tissue is the thin membrane,

FAT VESICLES-after Todd and Bowman, assuming the polyhedral form from pressure against one another.

thrown into closed vesicles, or cells; the fat is what these vesicles, or cells, contain. The tissue, or membrane, is about the one-twenty-thousandth part of an inch in thickness, and is quite transparent; it is of the simplest structure, and incapable of further mechanical analysis. Each vesicle is a distinct organ in itself, varying from one-three-hundredth to one-eight-hundredth part of an inch in diameter.

The fat itself is a form of oil, resolvable into stearine, oleine, and margarine.

Fat is extensively diffused through the animal kingdom. It exists not only in perfect insects, but also in the larvæ. It is found in molluscs. In all the tribes of vertebrated animals it is met with. In many fishes it is found only in the liver,-as the cod, the whiting, the haddock, and the rays. In reptiles it exists chiefly in the abdomen. In the frog, toad, &c. it is found in long bands on each side of the spine. In birds it exists chiefly between the peritoneum and the abdominal muscles; also, however, in the bones of the extremities, particularly of the swimming tribes. In mammals generally it abounds, yet with some exceptions; for example, the hare, in which sometimes hardly a particle of fat is discoverable.

In the healthy human fœtus fat accumulates in considerable quantity after the middle of the period of gestation. The quantity of fat in a moderately fat man has been estimated at about one-twentieth of his weight.

The white fibrous tissue and the yellow fibrous tissue are not confined to the areolar texture. The white fibrous, or inelastic fibrous tissue constitutes the ligaments of the joints and skeleton, the tendons of the muscles, and the membranes termed fibrous membranes. The yellow fibrous tissue, or the elastic fibrous tissue, forms some structures of great importance in which clasticity is requisite, as in the ligamenta subflava of the spine, and various parts of the mechanism of the larynx and windpipe. A peculiar modification of this texture constitutes the middle or proper coat of the arteries.

Simple membrane, together with epithelium, or epidermis, constitutes the tegumentary surface of the body, internal and external, or the mucous surfaces and the integuments; while an epithelium, spread over expanded cellular tissue, constitutes the serous membranes, or the linings of the shut cavities.

In the higher animals the mucous and serous membranes are well distinguished from each other. The former line the open cavities of the body: one extends in man, for example, from the frontal sinus into the cavities of the nose, ear, and mouth, and descends by the windpipe to line the countless number of minute air-cells; on the other hand, it passes through the gullet to the stomach, and so through the small and great intestines to the extremity of the rectum. The second great mucous membrane may be

54

THE LIQUOR SANGUINIS AND RED CORPUSCLES.

described as commencing in the pelvis of the kidney; it descends through the ureters to the bladder, and from the urethra, in both sexes, is transmitted into the organs of generation. The first of these great mucous membranes is termed the gastro-pulmonic membrane; the second, the genito-urinary mucous membrane. Owing to the extreme minuteness of the air-cells, which the pulmonic mucous membrane lines, the area of that membrane far exceeds the whole extent of the surface of the body.

The serous membranes line the great shut cavities of the body. The peritoneum, or serous membrane of the abdomen, is the largest membrane of this class. The membrane itself is a shut sac, like a double nightcap. The sac within contains nothing but secretion, the secreting surface being everywhere in contact with itself, that is, with another portion of the same inner surface of the sac; the inner or secreting surface being everywhere free, that is, unattached, while the outer surface is called the surface of attachment, because it is at every point united by coalescence with adjacent organs or parts.

Besides the peritoneum, or serous membrane of the abdomen, the serous membranes of the human body are,—the pleura, forming two separate shut sacs within the chest ; the pericardium, or serous membrane of the heart, often termed fibro-serous, as having a fibrous layer in connexion with it; the serous membrane of the brain, the arachnoid membrane; the serous membrane of the testicle, the tunica vaginalis; to which may be added, the synovical membranes, or membranes of the joints, and the bursal membranes, in which the great tendons play.

THE BLOOD IN RED-BLOODED ANIMALS.

By a happy phrase the blood has been described as "circulating flesh," or chair coulant. It ranks with the fluids; but the term fluid in Physiology differs widely from its signification in Physics. The blood is water, containing a considerable portion of solid organic matter. Human blood is about five per cent. denser than water; that is, human blood is water charged with about five per cent. of organic solid matter. The heaviest part of the solid matter of the blood consists of what are termed red particles, cr the red corpuscles, and these it is possible to separate by filtration from the remaining part of the blood. To succeed in this experiment, however, the blood of an animal must be chosen, in which the blood-corpuscles are considerably larger than in the human blood. In the frog the blood-corpuscles are four times the size of those in the blood of mammals. If, then, the blood of a frog be placed on a filter of common white filtering paper, a transparent fluid passes through the filter, and the red particles remain on its upper surface. By this experiment the blood is actually divided into the two parts, to which, respectively, physiologists attach a particular value. In the language of modern authorities, the portion which remains on the upper surface of the filter is the vesicular part; that which passes through is the "liquor sanguinis," or blood plasma. The portion which passes through the filter, after a few minutes begins to coagulate. The coagulum, or clot, gradually contracts with an exudation of watery fluid, by which it remains surrounded. The part which coagulates is fibrine; the liquid part, or what is usually called the scrum, being subjected to a temperature considerably short of that of boiling water (160° Fahrenheit), forms another coagulum, which is found to be albumen, or nearly identical with white of egg. The watery fluid which remains over is called serosity. This serosity contains all the soluble salts of the blood, and nothing else but a little animal matter.

Such, then, is a brief outline of the constituents of the blood; and even in the so

THE INFLAMMATORY CRUST.

55

called white-blooded animals, the composition of the blood is very much the same, since the absence of colour depends less on the total deprivation of red particles, than on the small proportion of that constituent being present.

We are now prepared better to understand what happens when blood is drawn from a vein in the human body. After a few minutes, blood so drawn assumes on the surface the appearance of a jelly, from which, after a time, drops of watery fluid here and there begin to ooze out; these drops become more and more numerous, and finally unite, so as to cover the jelly-like surface with a layer of watery fluid. After a short time, the clot, of which the jelly-like surface is the upper part, is so surrounded with the exuded watery fluid, as to be entirely separated, in most cases, from the sides of the vessel. The clot, however, does not always preserve the same degree of consistence. It is sometimes large, soft, and flabby; at other times small and firm, almost leathery. It consists, as might be anticipated, from what has been already stated, of the red particles and fibrine, or that substance which spontaneously coagulates when the blood of a frog has been subjected to filtration. The coagulation, then, of the clot depends on the coagulation of the fibrine which it contains, and not at all on its remaining chief constituents, the colouring corpuscles. When the clot is examined from top to bottom by a perpendicular section, it shows, in most cases, the red colouring matter diffused throughout, yet plainly in larger proportion at the lowest part, to which, owing to their greater weight, they gravitate before the coagulum has acquired sufficient consistence to intercept their progress. The colouring matter near the upper surface is usually of a more intense red colour than that below, owing, doubtless, to the action of the atmospheric air, by which the dark colour of venous blood acquires the vermilion hue of arterial blood. In every case the clot retains within it a portion of serum, or of the watery part of the blood. When the fibrine coagulates more weakly than usual, a larger proportion of this watery part is retained, giving to the clot an unusually soft and flabby consistence. Hence, without taking into account the degree of consistence of the clot, the relative proportion of the clot to the serum cannot be estimated. Of two cases in which the proportions are alike, the clot will be large in that in which the coagulation is weaker, and small in that in which the coagulation is stronger; the apparent quantity of the serum being greater in the latter case, owing to the large proportion of it retained in the clot. When the clot is large, and at the same time very firm, the fibrine is both abundant and highly coagulable.

The surface of the clot is generally quite flat; in other cases it is remarkably concave, or cupped, as it is termed. And when it is cupped, it is most commonly covered with a more or less thin layer of a yellowish opaque jelly, well known to physicians by the various names of size, buffy coat, and inflammatory crust. This yellow or buff-coloured layer on the surface of the clot, as its last-mentioned name indicates, is regarded by physicians as marking an inflammatory state of the body in the person from whom the blood was drawn. This layer is composed of the fibrine of the blood, separated from the red particles on the surface of the blood just before the clot forms. The unusual tendency to separation between the fibrine and the colouring particles, in cases where the buff is to appear, may be discovered while the whole blood is still fluid, by placing the cup between the eye and the light, when thin films, not unlike oil upon water, of a dark colour, will be seen floating on the surface of the blood. These films are plainly layers of fibrine already separated, through which, owing to their tenuity, in most cases, the dark colour of the venous blood shines. When the buff is to be very thick, these layers of fibrine on the surface of the still fluid blood, being opaque, exhibit their natural

56

SMALL PROPORTION OF FIBRINE IN BLOOD.

yellow colour. At the same time that there is this greater tendency to a separation between the red particles and the particles of fibrine, it has also been observed that the red particles have an unusually great disposition to unite together in the form of rolls, like piles of coins.

The following table exhibits, from recent authorities, the mean relative proportions of the several chief constituents of human blood in the two sexes :

What particularly strikes us on glancing at the table, is the small proportion of fibrine and the large proportion of albumen, notwithstanding that fibrine appears to be the nutrient constituent of which the most important solids of the body stand chiefly in need. Nay, the proportion of fibrine stated in the table is even an exaggeration, since what are termed the colourless blood-corpuscles cannot be sufficiently detached from the fibrine. The large proportion of red corpuscles also creates surprise, since these corpuscles are not directly concerned, as far as is known, in the nutrition of the solids. By far the most abundant solid in mammals, like man, is the muscular flesh. This muscular flesh is almost entirely made up of fibrine, identical, or nearly identical, with that which exists, however sparingly, in the blood. Further, when the animal body is much exercised, the muscular tissue is that which must require the greatest amount of repair; since it plainly appears that every living act is attended with a chemical decomposition and consequent waste in the organ concerned. It is impossible, then, to suppose that the small proportion of fibrine existing in the blood should be the source of repair to the muscular system. The proportion of fibrine in the blood is no more than one-fifth per cent.; so that, if the whole blood of the body be estimated at twenty-five pounds, the quantity of fibrine will be the one-twentieth of a pound, or something more than five drachms. It will hardly be maintained that the small proportion of fibrine in the blood arises from its unceasing exhaustion by the nutrition of the muscular tissue, for, were this the case, fibrine would increase enormously in the blood, after a few days' complete repose from muscular action.

Is it probable, then, that the albumen of the blood supplies the waste of the muscular tissue by passing into fibrine, when it is attracted from the liquor sanguinis into that tissue? In this supposition there is no difficulty. We have seen that albumen is very nearly identical with fibrine in ultimate composition; and it is certain that the egg, out of which the chick is developed,—that is to say, fibrinous flesh as well as blood, membrane, and bone,—consists of nothing but albumen, a little oil, and some saline matter. Of albumen there is about seven per cent. in the human blood, or in the mass of the cir culating blood there is something less than two pounds of albumen. Even this quantity will not suffice to supply the waste of the muscular tissue long, not to speak of the other demands upon it, without being continually renewed by the addition of the products of digestion.

As the proportion of fibrine in the blood is not found to diminish under deficiency of

food, it has been conjectured that it is the result rather of the decomposition of the blood itself, or of some of the tissues, than that it is designed to sustain any share in nutrition. But this view is not yet sufficiently matured to permit of being dwelt upon in this place.

The whole subject of the red corpuscles of the blood still presents great difficulties. Many observations have been made upon these bodies throughout the animal kingdom; but the exact use which they serve in the living frame is still a problem. These corpuscles constitute about 14 per cent. of the whole mass of human blood, or there is about twice as much by weight of the red corpuscles in the blood as there is of albumen, and seventy times as much as there is of fibrine.

When a drop of human blood is placed under the microscope, nothing but an opaque

RED CORPUSCLES FROM HUMAN

BLOOD, magnified 400 diameters after Todd and Bowman.

a, viewed on the surface; c, in profile; b, aggregation of corpuscles in a roll.

க

b

RED CORPUSCLES OF THE OX,

magnified 400 diameter
after Todd and Bowman.
a, in their natural state; b,
altered by a menstruum of
higher density.

RED CORPUSCLES OF THE PIGEON,
magnified 400 diameters-
after Todd and Bowman.
a, unaltered, with two or three
colourless particles; b, treated
with acetic acid, which more
clearly developes the cell-
wall and nucleus.

mass is seen, owing to the crowded state of the fluid with red corpuscles; but when the drop is diluted with a weak solution of salt or of sugar, each corpuscle is seen detached from the rest. The fluid used to dilute the drop of blood must be, as nearly as possible, of the same specific gravity as the serum of the blood; if plain water is employed, the red corpuscles swell and burst. Each corpuscle is round and flat, like a piece of money; or, to speak more correctly, each corpuscle has the form of a double concave lens, the margin being thick and rounded, and the centre considerably thinner: their size in the human body varies from the three thousandth to the four thousandth

RED CORPUSCLE IN FISHESafter Wharton Jones.

a, lamprey; b, skate.

RED CORPUSCLES OF CRAB

after Wharton Jones. a, three granule cells; b, three nucleated cells.

part of an inch in diameter. In mammals generally the blood-corpuscles are similar in figure to those in man; but there is a considerable variety of size in different tribes of these animals. They are small in ruminants, in the Napu musk-deer being no more than the twelve-thousandth part of an inch in diameter. In the camel tribe, instead of being round, they are oval, as they are in birds, reptiles, and fishes. In reptiles the blood-corpuscles attain a large size.

In the frog, the red corpuscles consist of a delicate membrane forming a cell, within which is a granular nucleus. The nucleus is globular, and much smaller than the cell; and the space between the inner surface of the cell and the outer surface of the nucleus