FIG. 16.-Very small portion of Fig. 15 very highly magnified.

A. walls of capillaries; B. tissue of web lying between the capillaries; C. cells of epidermis covering web (these are only shown in the right-hand

comes from the other end. When, however, we tie a vein the state of things is reversed, the swelling taking place on the side farthest from the heart, &c. &c., showing that in the veins the blood flows from the capillaries to the heart.

But certain of the lower animals, the whole, or parts, of the body of which are transparent, readily afford direct proof of the circulation, the blood visibly rushing from the arteries into the capillaries, and from the capillaries into the veins, so long as the animal is alive and its heart is at work. The animal in which the circulation can be most conveniently observed is the frog. The web between its toes is very transparent, and the particles suspended in its blood are so large that they can be readily seen as they slip swiftly along with the stream of blood, when the toes are fastened out, and the intervening web is examined under even a low magnifying power (Figs. 15 and 16).

and lower part of the field; in the other parts of the field the focus of the microscope lies below the epidermis); D. nuclei of these epidermic cells; E. pigment cells contracted, not partially expanded as in Fig. 15; F. red blood-corpuscle (oval in the frog) passing along capillary-nucleus not visible; G. another corpuscle squeezing its way through a capillary, the canal of which is smaller than its own transverse diameter; H. another bending as it slides round a corner; K. corpuscle in capillary seen through the epidermis; I. white blood-corpuscle.

LESSON III.

THE BLOOD AND THE LYMPH.

I. IN order to become properly acquainted with the characters of the blood it is necessary to examine it with a microscope magnifying at least three or four hundred diameters. Provided with this instrument, a hand lens, and some slips of thick and thin glass, the student will be enabled to follow the present Lesson.

The most convenient mode of obtaining small quantities of blood for examination is to twist a piece of string, pretty tightly, round the middle of the last joint of the middle, or ring finger, of the left hand. The end of the finger will immediately swell a little, and become darker coloured, in consequence of the obstruction to the return of the blood in the veins caused by the ligature. When in this condition, if it be slightly pricked with a sharp clean needle (an operation which causes hardly any pain), a good-sized drop of blood will at once exude. Let it be deposited on one of the slips of thick glass, and covered lightly and gently with a piece of the thin glass, so as to spread it out evenly into a thin layer. Let a second slide receive another drop, and, to keep it from drying, let it be put under an inverted watch-glass or wine-glass, with a bit of wet blotting-paper inside. Let a third drop be dealt with in the same way, a few granules of common salt being first added to the drop.

2. To the naked eye the layer of blood upon the first slide will appear of a pale reddish colour, and quite clear and homogeneous. But on viewing it with even a pocket lens its apparent homogeneity will disappear, and it will

look like a mixture of excessively fine yellowish-red particles, like sand, or dust, with a watery, almost colourless, fluid. Immediately after the blood is drawn, the particles will appear to be scattered very evenly through the fluid, but by degrees they aggregate into minute patches, and the layer of blood becomes more or less spotty.

The "particles" are what are termed the corpuscles of the blood; the nearly colourless fluid in which they are suspended is the plasma.

The second slide may now be examined. The drop of blood will be unaltered in form, and may perhaps seem to have undergone no change. But if the slide be inclined, it will be found that the drop no longer flows; and, indeed, the slide may be inverted without the disturbance of the drop, which has become solidified, and may be removed, with the point of a penknife, as a gelatinous mass. The mass is quite soft and moist, so that this setting, or coagulation, of a drop of blood is something very different from its drying.

On the third slide, this process of coagulation will be found not to have taken place, the blood remaining as fluid as it was when it left the body. The salt, therefore, has prevented the coagulation of the blood. Thus this very simple investigation teaches that blood is composed of a nearly colourless plasma, in which many coloured corpuscles are suspended; that it has a remarkable power of coagulating; and that this coagulation may be prevented by artificial means, such as the addition of salt.

3. If, instead of using the hand lens, the drop of blood on the first slide be placed under the microscope, the particles, or corpuscles, of the blood will be found to be bodies with very definite characters, and of two kinds, called respectively the red corpuscles and the colourless corpuscles. The former are much more numerous than the latter, and have a yellowish-red tinge; while the latter, somewhat larger than the red corpuscles, are, as their name implies, pale and devoid of coloration.

4. The corpuscles differ also in other and more important respects. The red corpuscles (Fig. 17) are flattened circular disks, on an average 20th of an inch in diameter, and having about one-fourth of that thickness. It follows that rather more than 10,000,000 of them will lie on a space

one inch square, and that the volume of each corpuscle does not exceed 120000000000th of a cubic inch.

The broad faces of the disks are not flat, but somewhat concave, as if they were pushed in towards one another. Hence the corpuscle is thinner in the middle than at the

FIG. 17.-RED AND WHITE CORPUSCLES OF THE BLOOD MAGNIFIED. A. Moderately magnified. The red corpuscles are seen lying in rouleaux; at a and a are seen two white corpuscles.

B. Red corpuscles much more highly magnified, seen in face; C. ditto, seen in profile; D. ditto, in rouleaux, rather more highly magnified; E. a red corpuscle swollen into a sphere by imbibition of water.

F. A white corpuscle magnified same as B.; G. ditto, throwing out some blunt processes; K. ditto, treated with acetic acid, and showing nucleus magnified same as D.

H. Ked corpuscles puckered or crenate all over, 7. Ditto, at the edge only,

edges, and when viewed under the microscope, by transmitted light, looks clear in the middle and darker at the edges, or dark in the middle and clear at the