The red discs were discovered in the human blood by Leuwen-hoek, about 1673. They give the red color which characterizes the blood of all vertebrated animals (except the amphioxus), but are not found in the blood of the invertebrata, which only contains colorless cells. When the blood of the invertebrates has a color it owes it to the fluid, not to the corpuscles. The individual discs when viewed singly under the microscope appear to be pale orange, but when in masses the red becomes apparent.

The shape of the corpuscles differs in different classes of animals. In man and all other mammalia they are discs, concave on each side and rounded off at the margin. The only class of mammals which forms an exception to this rule is the camel-idae, whose red corpuscles are elliptical in shape, like those of non-mammalian vertebrates.

The corpuscles of birds, amphibia and fish are flattened, elliptical plates, slightly convex on each side, and containing a distinct oval nucleus in their centre.

The size of the corpuscles varies greatly in different classes of animals, but is strikingly constant in the same class. A glance at the following diagram, in which the corpuscles are drawn to scale, will give an idea of their relative sizes, in examples of the different classes of animals, and will make the following points more rapidly obvious than mere description.

The size of the animal has no general relation to the size of the corpuscles. The human red discs are of a fair average size when compared with those of other mammals, and therefore man's blood cannot be distinguished from that of the other mammalia. The mammalian corpuscles are, on the whole, small when compared with those of the other vertebrates. The batrachians are distinguished by the great size of the corpuscles. Those of the Amphiuma tridactylum are visible to the naked eye.

Diagram of the relative sizes of red corpuscles of different animals.

Fig.99. Diagram of the relative sizes of red corpuscles of different animals. The measurements below are in fractions of a millimetre.

1. Amphiuma,....... 1/16 x 1/30

2. Proteus,........ 1/18 x 1/44

3. Frog........... 1/45 x 1/66

4. Pigeon,......... 1/225 x 1/120

5. Elephant,........ 1/108

6. Man,........... 1/126

7. Dog............ 1/139

8. Horse, .......... 1/181

9. Goat,........... /253

10. Musk Deer,........ 1/483.

The following measurements are given by Welcker for the human discs: -

Diameter,. . ..0077 of a millimetre (7.7/u*)= 1/3200 of an inch. Thickness, . ..0019 of a millimetre (1.9//) = 1/12400 of an inch. Volume,. .000000077 of a cubic millimetre. Surface,. . .000128 of a square millimetre.

The last measurement would give a surface of about 2816 square metres for the corpuscles of an adult. A surface of 11 square metres is exposed every second in the lungs for the absorption of oxygen.

When circulating in the vessels, or immediately after removal, the red corpuscles are very soft and elastic, being bent and altered in shape by the slightest pressure, and easily stretched to twice their diameter. But the moment pressure or traction is removed, they return to their normal biconcave disc shape if the medium in which they lie continue of the normal density.

Changes take place in the blood shortly after it is removed from the body, which seem to be associated with the loss of function (death) of the red discs, as shown by their rapid destruction if reintroduced into the circulation.

The changes are checked by cold and facilitated by heat, a temperature above that of the body causing them to take place almost immediately. Associated with the loss of function of the discs is observed a change accompanied by an apparent increase of adhesiveness, which causes them to stick together, commonly adhering by their flat surfaces, so as to form into rolls, like so many coins placed side by side. That this adhesion is not a mere physical process, independent of the chemical properties of the corpuscles themselves, seems proved by the following facts: (1) It does not occur immediately when the blood is drawn, and disappears after a few hours without the addition of reagents; (2) while the blood is in the living vessels under normal conditions there is no adhesion, but this soon appears when any standstill in the circulation takes place - as in inflammation; (3) it does not occur when saline solutions are added to the blood. It seems to be dependent upon a peculiar property of the discs, which only exists for a time coincident with the changes that accompany the death of the blood and the appearance of fibrin.

* The Greek letter /u. is used by histologists to denote 1/1000 of a millimetre, which is taken as a convenient unit of measurement.

The shape of the discs changes when the density of the medium in which they are suspended is altered. When the density is reduced, as by the addition of water, they swell and become spherical, and break up the rouleaux; the coloring matter at the same time becoming dissolved in the medium. (Fig. 100.) When the density is increased by slight evaporation, or the addition of salt solution about 1 per cent., they cease to be concave, and become crenated or spiked like the green fruit of the horse-chestnut. (Fig. ioi.) The addition of strong syrup causes the corpuscles to shrivel and assume a great variety of peculiar bent or distorted forms. (Fig. 102.) Elevation of temperature or repeated electric shocks causes a peculiar change in shape, but since the change is associated with the death of the element, it cannot be attributed to vital activity comparable with that seen in the white cells.

Microscopic appearance of the blood after the addition of distilled water.

Fig. 100. Microscopic appearance of the blood after the addition of distilled water. Red Corpuscles become colorless or pale, separate and spherical. The white are seen to be swollen, round and granular with clear nuclei.

Showing effect of evaporation. Six Red Corpuscles crenated. (w) White cell changing shape.

Fig. 101. Showing effect of evaporation. Six Red Corpuscles crenated. (w) White cell changing shape.

The discs show no signs of structure under the microscope: they are perfectly homogeneous, transparent bodies, of a pale orange color, all efforts to demonstrate the limiting membranes formerly supposed to surround them having failed. Their behavior when certain reagents are added to the blood shows that the corpuscles have two constituents: (1) the coloring matter, Oxyhemoglobin; and (2) the Stroma. The coloring matter may be removed from the corpuscle, as above stated, by water, and leaves a perfectly colorless transparent foundation or groundwork, which appears to be in some way porous, so as to hold the coloring matter in its interstices. The effect on the naked-eye appearance of the blood produced by the removal of the coloring matter from the stroma is to alter the color and increase the transparency of the fluid. The oxyhemoglobin now forms a transparent, dark-red, lakey solution, and the corpuscles, being quite colorless, are practically invisible. This transparency of the fluid does not depend on any change in the oxyhemoglobin, but merely on its being dissolved out of the discs, which become transparent and can no longer reflect the light. This process, which is commonly spoken of as rendering the blood "lakey," may be brought about by the following means: (1) The addition of about 1/4 its bulk of distilled water, to wash the coloring matter out of the stroma, which may then be rendered visible by a weak solution of iodine. (2) By the addition of chloroform, ether, or alkalies. (3) By passing repeated strong induction shocks through the blood. (4) By rapidly freezing and thawing the blood several times.

Red Corpuscles shriveled by the addition of strong syrup, (w) White Corpuscle.

Fig. 102. Red Corpuscles shriveled by the addition of strong syrup, (w) White Corpuscle.

Blood Corpuscle after the addition of tannic acid. (1/2%).

Fig. 103. Blood Corpuscle after the addition of tannic acid. (1/2%).

All these processes produce the same effect; viz., the red matter leaves the stroma and passes into solution without producing a marked change in either, as if the solution depended upon the destruction of some vital relationship between the stroma and the oxyhemoglobin which prevented the diffusion of the latter in the living blood.

Solutions of urea, bile, acids and heat of about 6o° C. seem to destroy the discs, and thus remove the coloring matter. Carbolic, boracic and tannic acids cause the coloring matter to coagulate and localize itself either at the centre or margin of the corpuscle. (Fig. 103).

The number of discs in the blood of man is enormous, namely, in a cubic millimetre of blood, about 5 millions for males and 4 1/2, millions for females, or about 250,000 millions for one pound of blood. The number varies much, not only in disease, but also as a result of the many physiological processes, such as changes in the amount of plasma, brought about by pressure differences, etc. In order to count the corpuscles the following method is employed; The blood is diluted with artificial plasma to 100 or 1000 times its volume, and the corpuscles in a portion of the mixture carefully measured off by a capillary tube, and counted. This operation requires great care and delicate apparatus. One 20 of the best-known methods is that of Malassez, the details of which are as follows: -

Malassez' Apparatus for the Enumeration of Blood Corpuscles.

Fig.104. Malassez' Apparatus for the Enumeration of Blood Corpuscles. A, Measuring and mixing pipette. B, Flattened and calibrated capillary lube.

Blood is drawn into the capillary tube of a specially prepared delicate pipette (Fig. 104, a) up to a mark which indicates 1/100 part of the capacity of the pipette. This known quantity of blood is then washed into the bulb of the pipette by drawing up artificial serum to fill the bulb, where the fluids are mixed by shaking about a glass bead contained in its cavity. Some of this mixture is then allowed to pass into a flattened capillary tube of known capacity fixed on a slide, and the number of corpuscles in a given length of this tube is carefully counted at two or three places. The important question, how much oxyhemoglobin exists in a given sample of blood, can be determined by diluting some of it until the color equals that of a standard solution of known strength.

The appearance presented by the Capillary lube of Malassez Apparatus when filled with diluted blood.

Fig.105. The appearance presented by the Capillary lube of Malassez Apparatus when filled with diluted blood and examined under a microscope magnifying 100 diameters, provided with an eye-piece micrometer.