From the earliest ages the blood has been held to be one of the most important constituents of the body, for it was natural, when death was seen to follow alike in animals and in man the infliction of a small wound (providing it opened a large vessel), to believe that as the blood drained away, the life itself was leaving the body. The practice of strict Jews of all periods of their history, acting up to the command: " But flesh, with the life thereof, which is the blood thereof, shall ye not eat", is evidence of the strong impress the constant association of loss of blood and of life has made on the mind of man. In many points of view it is indeed the river of life, for its flow supplies to all parts of the body the materials requisite for their development, maintenance, and repair, whilst it is in addition charged with the vivifying oxygen it has absorbed at the lungs. On the other hand, it carries away from each part the products of disintegration and decay, and conducts them to the organs by which they may be severally eliminated from the system, the most important being the carbon dioxide, which is discharged at the lungs, and the urea, which is excreted or thrown off by the kidneys.

The blood of the horse, like that of other mammals, is of a deep-red colour, but brighter in the arteries than in the veins. The taste is mawkish, the odour faint but peculiar, and its reaction to test-paper invariably alkaline. Its specific gravity is about 1.061. It is clammy or slippery to the touch, and is remarkably opaque, transmitting but little light even in thin layers. It is in constant movement in the body. The quantity of the blood contained in a horse of average size is estimated to be about one-eighteenth of its body weight, or from 40 to 45 lbs., and it is considered that one-fourth is contained in the heart and larger blood-vessels, one-fourth in the muscles, one-fourth in the liver and intestines, and the remaining fourth in the other organs of the body. To the unassisted eye, the blood as it issues from a wound appears to be perfectly homogeneous, but when examined with a microscope of moderate power it is seen to be composed of a transparent fluid named the plasma, in which are suspended a large number of corpuscles (fig. 184). The existence of corpuscles in blood was first noticed in the hedgehog by the celebrated Italian physician and anatomist Malpighi in the year 1661, who thought they were globules of fat. They had previously been seen, in 1658, by the Dutch anatomist Swammerdam in the frog, but this investigator lost the credit of the discovery in consequence of his failing to publish his observations. The real merit of recognizing the corpuscles as constant and essential elements of the blood is due to another Dutch microscopist named Leeuwenhoek, who in 1673 observed and described them in man and many other animals. Great attention has been bestowed upon them by numerous observers, and three chief forms of corpuscles have been distinguished, named, respectively, red corpuscles, white corpuscles, and blood platelets.

Blood Corpuscles.

Fig. 184. - Blood Corpuscles.

A, Coloured Blood Corpuscles adhering together in Columns (rouleaux). B, Coloured Corpuscle, showing concave surface.

C, The same seen edgewise. D, E, F, Colourless Corpuscles.

The red corpuscles are pale-yellow circular discs, each resembling a coin, with edges thicker than the central part, and they are hence said to be biconcave. Their consistence is like that of moderately firm jelly. They have no nucleus. The transverse diameter is about 1/3000 inch, and their thickness about 1/12000 inch. They are a little heavier than the fluid in which they are suspended, and consequently have a tendency to fall to the bottom of the vessel when blood is removed from the body. This disposition to gravitate may be observed in the living animal, since if blood be gently drawn with a small syringe from the upper and lower parts of a large horizontal vein - like the external jugular or neck vein when the animal is recumbent - the number of the corpuscles contained in the specimen taken from the lower part of the vein will be found to be much greater than in the specimen taken from the upper part. The number of the corpuscles in the body of a horse is inconceivably great, but they may be counted in small quantities of blood which have been diluted with water, and it has been ascertained that there are no less than five or six millions in a cube 1/25 inch on the side, which would be represented by a very small drop. In every hundred parts of the red corpuscles there are about seventy parts of water and thirty parts of solids, and if the solids be examined after the water has been evaporated, every hundred parts are found to consist of eighty-eight parts of haemoglobin, ten parts of proteid substance of the nature of globulin, and two parts of lecithin and cholesterin. The haemoglobin then is the most abundant, as it is the most important, constituent of the blood. It is to it that the blood owes its colour, and it possesses ^averal remarkable properties. In the first place, it is one of the prime factors in the process of respiration, being the carrier of oxygen between the air and the tissues, combining with this gas in the lungs, but holding it with so weak a grasp as to surrender it to the tissues during the brief period that it is in proximity with them whilst traversing the more minute or capillary vessels. It is possible, although it has not as yet been proved, that haemoglobin presents similar relations with carbon dioxide, taking up that gas in the tissues, in exchange for the •oxygen with which it parts, and permitting its escape at the lungs in exchange for the oxygen it there absorbs. In this case it would serve as a carrier for both oxygen and carbon dioxide, its relations to each gas being governed by the degree of chemical affinity between the haemoglobin and the gas and by the tension of the gas at the moment of exposure to it. Experiment has shown that every ten grains of haemoglobin is able to absorb about a cubic inch of oxygen gas. Its relation to carbon dioxide is less accurately known. Another peculiarity of haemoglobin is its capability of forming crystals (fig. 185), some of the forms of which are here shown. The shape of these crystals in the horse is prismatic, as is usual in mammals. They are soluble in water. When by various means, such as freezing and again thawing, or by the addition of a little chloroform or ether to fresh blood, the haemoglobin is rendered soluble in the plasma, the blood retains its colour, but becomes transparent like port wine. It is then named laky blood. The chemical composition of haemoglobin is extraordinarily complex, one of the latest observers giving the formula -