edges, according to circumstances. When, on the other hand, the disks roll over and present their edges to the eye, they look like rods. All these varieties of appearance may be made intelligible by turning a round biscuit or muffin, bodies similar in shape to the red corpuscles, in various ways before the eye.

The red corpuscles are very soft, flexible, and elastic bodies, so that they readily squeeze through apertures and passages narrower than their own diameters, and immediately resume their proper shapes (Fig. 16, G.H.). The exterior of each corpuscle is denser than its interior, which contains a semi-fluid, or quite fluid matter, of a red colour, called hæmoglobin. By proper processes this may be resolved into an albuminous substance sometimes called globulin, and a peculiar colouring matter, which is called hæmatin. The interior substance presents no distinct structure.

From the density of the outer as compared with the inner substance of each corpuscle, they are, practically, small flattened bags, or sacs, the form of which may be changed by altering the density of the plasma. Thus, if it be made denser by dissolving saline substances, or sugar, in it, water is drawn from the contents of the corpuscle to the dense plasma, and the corpuscle becomes still more flattened and very often much wrinkled. On the other hand, if the plasma be diluted with water, the latter forces itself into and dilutes the contents, of the corpuscle, causing the latter to swell out, and even become spherical; and, by adding dense and weak solutions alternately, the corpuscles may be made to become successively spheroidal and discoidal. Exposure to carbonic acid gas seems to cause the corpuscles to swell out; oxygen gas, on the contrary, appears to flatten them.

5. The colourless corpuscles (Fig. 17, a a, F. G. K.) are larger than the red corpuscles, their average diameter being 2500th of an inch, They are further seen, at a glance, to differ from the red corpuscles by the extreme irregularity of their form, and by their tendency to attach themselves to the glass slide, while the red corpuscles float about and tumble freely over one another.

A still more remarkable feature of the colourless

corpuscles than the irregularity of their form is the unceasing variation of shape which they exhibit. The form of a red corpuscle is changed only by influences from without, such as pressure, or the like; that of the colourless corpuscle is undergoing constant alteration, as the result of changes taking place in its own substance. To see these changes well, a microscope with a magnifying power of five or six hundred diameters is requisite : and, even then, they are so gradual that the best way to ascertain their existence is to make a drawing of a given colourless corpuscle at intervals of a minute or two. This is what has been done with the corpuscle represented in Fig. 18, in which a represents the form of the corpuscle when first observed; 6, its form a minute afterwards; c, that at the end of the second; d, that at the end of the third; and e, that at the end of the fifth minute.

FIG. 18.-SUCCESSIVE FORMS ASSUMED BY COLOURLESS CORPUSCLES OF HUMAN BLOOD. (Magnified about 600 diameters.)

The interval between the forms a, b, c, d was a minute; between d and e two minutes; so that the whole series of changes from a to e took five minutes.

Careful watching of a colourless corpuscle, in fact, shows that every part of its surface is constantly changing-undergoing active contraction, or being passively dilated by the contraction of other parts. It exhibits contractility in its lowest and most primitive form.

6. While they are thus living and active, no correct notion can be formed of the structure of the colourless corpuscles. By diluting the blood with water, or, still better, with water acidulated with acetic acid, the corpuscles are killed, and become distended, so that their real nature is shown. They are then seen to be spheroidal bags, or sacs, with very thin walls; and to contain in their interior a fluid which is either clear or granular, together with a spheroidal vesicular body, which is called

the nucleus (Fig. 17, K). It sometimes, though very rarely, happens that the nucleus has a red tint.

The sac-like colourless corpuscle, with its nucleus, is what is called a nucleated cell. It will be observed that it lives in a free state in the plasma of the blood, and that it exhibits an independent contractility. In fact, except that it is dependent for the conditions of its existence upon the plasma, it might be compared to one of those simple organisms which are met with in stagnant water, and are called Amabæ.

7. That the red corpuscles are in some way or other derived from the colourless corpuscles may be regarded as certain but the steps of the process have not been made out with perfect certainty. There is very great reason, however, for believing that the red corpuscle is simply the nucleus of the colourless corpuscle somewhat enlarged; flattened from side to side; changed, by development within its interior of a red colouring matter; and set free by the bursting of the sac or wall of the colourless corpuscle. In other words the red corpuscle is a free nucleus.

The origin of the colourless corpuscles themselves is not certainly determined; but it is highly probable that they are constituent cells of particular parts of the solid substance of the body which have been detached and carried into the blood, and that this process is chiefly effected in what are called the ductless glands (Lesson V. §27), from whence the detached cells pass, as lymphcorpuscles, directly or indirectly, into the blood.

The following facts are of importance in their bearing on the relation between the different kinds of corpuscles :

(a) The invertebrate animals,' which have true bloodcorpuscles, possess only such as resemble the colourless corpuscles of man.

(b) The lowest vertebrate animal, the Lancelot (Amphioxus), possesses only colourless corpuscles; and the very young embryos of all vertebrate animals have only colourless and nucleated corpuscles.

I Invertebrate animals are animals devoid of backbones, such as insects, snails, sea-anemones, &c. Vertebrate animals are fishes, amphibia, reptiles, birds, and mammals.

2 An embryo is the rudimentary unborn young of any creature.

(c) All the vertebrated animals, the young of which are born from eggs,1 have two kinds of corpuscles-colourless corpuscles, like those of man, and large red-coloured corpuscles, which are generally oval, and further differ from those of man in presenting a nucleus. In fact, they are simply the colourless corpuscles enlarged and coloured.

(d) All animals which suckle their young (or what are called mammals) have, like man, two kinds of corpuscles; colourless ones, and small coloured corpuscles-the latter being always flattened, and devoid of any nucleus. They are usually circular, but in the camel tribe they are elliptical. And it is worthy of remark that, in these animals, the nuclei of the colourless corpuscles become elliptical.

(e) The colourless corpuscles differ much less from one another in size and form, in the vertebrate series, than the coloured. The latter are smallest in the little Musk Deer, in which animal they are about a quarter as large as those of a man. On the other hand, the red corpuscles are largest in the Amphibia (or Frogs and Salamanders), in some of which animals they are ten times as long as in

man.

8. As the blood dies, its several constituents, which have now been described, undergo marked changes.

The colourless corpuscles lose their contractility, but otherwise undergo little alteration. They tend to cohere neither with one another, nor with the red corpuscles, but adhere to the glass plate on which they are placed.

It is quite otherwise with the red corpuscles, which at first, as has been said, float about and roll, or slide, over each other quite freely. After a short time (the length of which varies in different persons, but usually amounts to two or three minutes), they seem, as it were, to become sticky, and tend to cohere; and this tendency increases until, at length, the great majority of them become applied face to face, so as to form long series, like rolls of coin. The end of one roll cohering with the sides of another, a network of various degrees of closeness is produced (Fig. 17, A.).

The corpuscles remain thus coherent for a certain length of time, but eventually separate and float freely I These are fishes, amphibia, reptiles, and birds.

again. The addition of a little water, or dilute acids or saline solutions, will at once cause the rolls to break up.

It is from this running of the corpuscles together into patches of network that the change noted above in the appearances of the layer of blood, viewed with a lens, arises. So long as the corpuscles are separate, the sandy appearance lasts; but when they run together, the layer appears patchy or spotted.

The red corpuscles rarely, if ever, all run together into rolls, some always remaining free in the meshes of the net. In contact with air, or if subjected to pressure, many of the red corpuscles become covered with little knobs, so as to look like minute mulberries-an appearance which has been mistaken for a breaking up, or spontaneous division, of the corpuscles (Fig. 17, H. K.).

9. There is a still more remarkable change which the red blood-corpuscles occasionally undergo. Under certain circumstances, the peculiar red substance which forms the chief mass of their contents, and which has been called hæmoglobin (from its readily breaking up into globulin and hæmatin, § 6), separates in a crystalline form. In man, these crystals have the shape of prisms; in other animals they take other forms. Human blood crystallizes with difficulty, but that of the guinea-pig, rat, or dog much more easily. The best way to see these bloodcrystals is to take a little rat's blood, from which the fibrin has been removed, shake it up with a little ether, and let it stand in the cold for some hours. A sediment will form at the bottom, which, when examined with the microscope, will be found to consist of long narrow crystals. Crystallization is much assisted by adding after the ether a small quantity of alcohol.

10. When the layer of blood has been drawn ten or fifteen minutes, the plasma will be seen to be no longer clear. It then exhibits multitudes of extremely delicate filaments of a substance called Fibrin, which have been deposited from it, and which traverse it in all directions, uniting with one another and with the corpuscles, and binding the whole into a semi-solid mass.

It is this deposition of fibrin which is the cause of the apparent solidification, or coagulation, of the drop upon the second slide; but the phenomena of coagula

F