This work is, of course, designed more for con sultation than for systematic reading. Never theless it has been considered advisable to take up one part of the body after another in a regular order, so that, if any one chooses to read the sections on ANATOMY AND PHYSIOLOGY in the order in which they are given, a general and connected account of the whole body in its structure and workings will be obtained. It is of value for the understanding of the subject of this section to consider, for a moment, what stage has been reached in the regular study of the body. In Section X. we discussed the process of digestion, and perceived that the whole end and aim of that process was to pre pare material fit to enter the blood and main tain its quality and quantity. The considera tion of the lymphatics and blood glands, in Section XII., showed that the blood received supplies froM lymphatic vessels of lymph pre pared by lymphatic glands, and had also addi tions made to it by the blood glands, notably the spleen. We thus perceive that the blood, which is being continually drained by the de mands made on it by the whole body for its nourishing material, has two main sources of supply, namely, first and chiefly, the food, and secondly, the lymphatics and blood glands. We shall see in Section XVI. that this does not exhaust the sources whence the blood draws supplies, and that the lungs are the channel by which a substance is conveyed to the blood, not second in importance to what is received by the alimentary canal, the substance oxygen gas, namely.
Such being the sources of the blood, our next question is as to its nature.
Microscopical Characters of Blood.— The microscope reveals much as to the nature of the blood. Blood is prepared for examina tion by the microscope in a very simple way, The twisted corner of a handkerchief is wound tightly round the end joint of a finger. This obstructs the flow of blood, and the point of the finger becomes purple and congested. A smart dab with a clean needle draws a drop of blood at once. A small quantity is got on the centre of a slide, such as is used for microscopic purposes, by making the slide touch the drop, and it is immediately covered with a very thin circle of glass (a cover glass). The slide is then put on the stage of a microscope and examined.
Under a moderately high magnifying power the appearance represented in Fig. 131 is seen. The blood is evidently not altogether fluid. It , contains small bodies in great numbers, which are floating in a liquid. Contrary to what would be expected, the liquid is of a light straw colour, indeed almost colourless. The small bodies, however, are coloured. They appear red when seen in layers, but singly they are yellow. The fluid is called plasma, or liquor sanguinis (liquid of the blood); the small bodies are the red corpuscles (small body, Latin corpus, a body) or cells of the blood. If the preparation be quickly examined, or if the cover glass be gently disturbed with a needle, the corpuscles will be seen separate from one another, but they quickly run to gether to form rows or rouleaux, like piles of coin, as represented in the figure. This is owing to their shape. If one be carefully examined as it lies on its edge, it presents the appearance shown at a in Fig. 131, thinner at the middle than at either end. If one be seen lying on its face its appearance is as repre sented by 1, of the same figure. It is circular, and the centre is dark, while the edge is clear. If the focus of the microscope be altered, the centre becomes clear and the edge dark. In other words the two surfaces of the corpuscle are not flat, they are hollowed towards the centre, so that the corpuscle is thinner at the centre than at the margins. The body is thus hollowed on each surface, is, in a word, a biconcave disc. On a casual glance the red corpuscles are the only bodies seen in the fluid, but, on looking carefully, other bodies slightly larger and few in number are perceived. They are seen here and there in the spaces formed by the rows of red cells. They keep separate from one another, are white, and contain little dark granules in their interior. They are the white or colourless corpuscles of the blood, and are also termed leucocytes (Greek leukos, white, and khtos, a cell). They are represented in Fig. 131, c. There are usually not more than three or four seen in the field. There is in healthy human blood, on an average, 1 white blood corpuscle for every 600 to 1200 red ones. They are irregularly globular in shape.