coloured corpuscles, the more numerous, and the white corpuscles, far fewer in number, 2 or 3 of the latter among 1,000 of the former, so that they require to be looked for with some little care. All the more so that the tint of the coloured corpuscles is faint, they are not far from colourless, and it is only by their great number that the strong red colour of blood is produced. On still more careful examination with very high powers, very small colourless corpuscles-about 2μ in diameter, 2 or 3 to every 100 red corpuscles may be distinguished; these are called blood-platelets or hæmatoblasts. A 10 Human blood-corpus view. cles, side and front single coloured corpuscle seen flat is circular, and, as the microscope is focussed up and down with the fine cles, side and front Frog's blood-corpus- adjustment, appears now as a dark centre surrounded by a light ring, now as a light centre surrounded by a dark ring. This appearance is not due to a nucleus-human bloodcorpuscles, in common with those of all mammalia, have no nucleus. FIG. 3. The corpuscles are drawn to the same scale, viz. x 1000. 1μ or 10 mm. of actual length is represented by 1 mm.; the human corpuscle as sketched above is 7 mm. in diameter, i.e. its actual diameter is 7μ; the frog's cor puscle is 20μ by 11. The corpuscles of mammalia resemble those of man, but differ in size. Average magnitudes are: elephant, 94; dog, rat, 7u; cat, horse, ox, 6u; goat, sheep, 5μ. But the corpuscles of man, averaging 7u, may range from as low as 5μ to as high as 9μ. but to the cupped shape of the corpuscles; a single corpuscle seen sideways looks something like a dumb-bell, and the two views combined show that the actual shape of the corpuscle is that of a disc cupped on both surfaces- -a bi-concave disc; these discs frequently run together and adhere to each other in longer or shorter rolls like piles of coins; if distilled water be added to blood, water enters the red corpuscles and causes them to become globular and then burst; if strong solution of salt be added, water leaves the corpuscles, which become shrivelled and crenated; these experiments indicate that the corpuscle is surrounded by a film. The blood of amphibia, reptiles, fishes, and birds has corpuscles different from those of mammalian blood in shape and structure. Of all these lower classes the red corpuscles are oval, convex on both sides, and their central part differs in structure from the remainder of the corpuscle, constituting a nucleus. Various instruments have been devised for enabling an estimate to be formed of the number of corpuscles present in a given pige 10257 volume of blood; the principle of all such instruments is to furnish the observer with a very minute cubic space under the microscope, in which it is possible to see and count the corpuscles of blood diluted a given number of times. The form of instrument most used in this country has been called a hemacytometer (Gowers); the blood is diluted 200 times, and observed through the microscope in a cell millimeter deep, and divided into millimeter squares: the cubic space of each division is thus millimeter, and the number of corpuscles in one division multiplied by 100,000 gives, therefore, the number of corpuscles per cubic millimeter. Normally, this number is 4,000,000 to 5,000,000 (40 to 50 in each square); abnormally, in extreme anæmia, it may be as low as 1,000,000 to 2,000,000 (10 to 20 in each square). The white corpuscles, or leucocytes, are of all shapes while they are alive, and their shapes are constantly, but very slowly, changing; ultimately-i.e. as they die-they assume a globular form. Many of them break up and vanish. They are of various sizes; some are no larger than the coloured corpuscles, some are even smaller, some are considerably larger; and of the larger leucocytes, some contain a coarsely granular substance, staining deeply with acid dyes, such as eosin, others are homogeneous with a single large nucleus, others still-and these are the most important and numerous-have two or three ele, staining uchi deeply with neutral dyes, such as methyl blue or fuchsin. These last may be regarded as typical leucocytes, and exhibit the various life-characters that will presently be described (p. 25). Chemical composition.-100 grammes of blood consist of about 80 grammes of water and 20 grammes of solids, some in solution, some in suspension, and contain 60 cubic centimeters of gases, some in solution, some in loose chemical combination with other constituents of the blood. The twenty grammes of solids comprise :Hæmoglobin Water 80 Solid 10 grammes The important proteid, fibrinogen, forms only a very small proportion of the total proteid; although the fibrin which it forms appears of considerable bulk when it is separated by whipping, yet its actual weight is small-1,000 grammes of blood yield between 2 and 4 grammes of fibrin. The hemoglobin is contained in the coloured corpuscles, which owe to this substance their function as oxygen-carriers to the tissues. The proteids comprise serum-albumin and serum-globulin in about equal proportions dissolved in the plasma. Physically considered, 100 grammes of horse's blood consist of about:: The average amount of hæmoglobin in human blood is somewhat higher-between 12 and 15 per 100. The 66 grammes of plasma comprise : The chief salts of the blood are chlorides and phosphates, combined with godium and potassium. Sodium chloride preponderates in the plasma; potassium phosphate preponderates in the corpuscles. Other salts present are sulphates and sodium bicarbonate (NaHCO3). Fat, sugar, and urea are constantly present in the blood, but in small quantities only. Fat is present in variable quantity; after a meal of fat it may be so abundant as to form a scum on the surface of drawn blood; it remains, however, but a very short time in circulating blood, being separated by the tissues and stored in them; 1,000 grammes of blood contain from 2 to 6 grammes of fat. Sugar is present in small but constant quantity; 1,000 grammes of blood contain about 1 gramme of sugar. Urea is present in small quantity; 1,000 grammes of blood contain about to gramme. Plasma, serum, fibrinogen, fibrin. The fluid in which the corpuscles are suspended while the blood is uncoagulated is called the plasma, or liquor sanguinis. It differs from the fluid in which the clot floats after the blood has coagulated, i.e. the serum, in this important respect, that plasma contains a body called fibrinogen, while serum contains no fibrinogen; both fluids have a specific gravity of about, 1,030, and contain about the same quantity of proteids. Fibrinogen is, as its name implies, a body which becomes or gives rise to fibrin. It is the conversion of fibrinogen into fibrin which is the essential event in the coagulation of the blood. Fibrinogen in the plasma becomes fibrin, which pervades the fluid in all directions in the form of innumerable filaments; the plasma having lost fibrinogen, serum is left. The filaments of fibrin shrink, and, entangling in their meshes the corpuscles, sweep these from the serum, and form with them the clot, which gradually shrinks and squeezes out the serum. To FIG. 5.-PLAN OF CENTRIFUGE AS USED FOR THE SEPARATION OF PLASMA. Centrifugal Apparatus. Disc fitted with four long buckets pivoted so that they hang vertically when the disc is at rest, and assume a horizontal position as the disc revolves. Large test tubes containing the blood-mixture fit into the buckets. By centrifugal force, while the disc is revolving, the corpuscles collect towards the circumference, i.e. at the bottom of the test-tubes; the plasma is left nearer the centre, i.e. at the top of the test-tubes. Stationary. The buckets (represented in transverse section) hang vertically from the disc. Revolving. The buckets (represented in longitudinal section) swing out to a horizontal plane. obtain serum it is only necessary to allow freshly-drawn blood to coagulate without disturbance. To obtain fibrin, blood is whipped with a bundle of twigs as soon as it is drawn; the fibrin adheres to the twigs and forms a stringy mass, which is to be subsequently washed in water and left under a running tap until almost white. The whipped blood from which fibrin has been separated is defibrinated blood. To obtain plasma it is necessary to prevent coagulation, so that the corpuscles may subside and leave an upper layer of plasma. To this end, the blood is allowed to flow into a vessel surrounded with ice, and containing a saturated solution of magnesium sulphate. After a few hours the corpuscles will subside if the vessel is left undisturbed, or if, instead of leaving the separation to take place by the action of gravity, the vessel is fixed to a horizontal wheel revolving at a high speed, the separation will be much accelerated by centrifugal force. It is convenient, but not absolutely necessary, to employ both ice and magnesium sulphate. From blood mixed at once with magnesium sulphate at ordinary temperature, plasma can be obtained by subsidence, or by the centrifuge. By cooling blood sufficiently rapidly, and leaving it to stand surrounded by melting ice, plasma may be obtained without magnesium sulphate. Horse's blood, which coagulates more slowly than that of other mammals, is the best kind of blood to choose for the purpose. Another convenient method of obtaining plasma is afforded by the property which commercial peptones possess of preventing coagulation when they are injected into the vessels (pp. 23 and 189). The blood of a dog so treated does not coagulate after it is drawn, and plasma may be obtained from it by the centrifuge. The upper part of the clot that forms in normal horse's blood is not red like the lower portions, but pale or buff-coloured. This difference is due to the fact that the red corpuscles of horse's blood cling together in little clumps which quickly subside from the upper part of the fluid, and the clot that forms subsequently is comparatively free from them. A buffy coat has also been noticed in human blood in inflammatory conditions; the coagulation of normal human blood is too rapid to allow of the formation of a buffy coat, but inflammatory blood coagulates more slowly, and the upper part of the clot is left pale. Defibrinated blood is opaque, and viewed by reflected light it is red. After an equal volume of distilled water has been added, it becomes translucent, and viewed by reflected light it is of a dark lake colour. The difference is owing to the action of water on the red corpuscles; the hæmoglobin has been dissolved, and, leaving the stroma, has become diffused throughout the fluid; the colourless stromata permit the transmission of light through the fluid, and less light is reflected from it. Hence it is translucent when viewed by transmitted light, dark lake-coloured when viewed by reflected light. Blood may be laked' by other means than water-e.g. by alternate freezing and thawing, or by the addition of chloroform. The composition of plasma, lymph, chyle, and serous fluids.— The proteids of blood are for the most part contained in the plasma, the corpuscles containing a comparatively small |