210 USES OF RED CORPUSCLES-LIQUOR SANGUINIS. carbonic acid which it gives-off by the same channel, vary, therefore, with the muscular exertion it makes. This variation is most easily observed and measured in Insects; and it is found in them to be enormous (§ 308). As, however, the blood of the Invertebrata does not contain these red particles, to which so important a function has been assigned, it may be asked, how the conveyance of oxygen to their tissues is provided for. The reply is very simple. In Insects, and other ARTICULATA which have active powers of motion, the air is conveyed to the tissues, not through the medium of the blood, but directly through air-tubes which convey it to every part of the body (§ 321). And in the MOLLUSCOUS classes, as among the Crustacea also, the nervo-muscular system forms. so subordinate a part of the general mass of the body, and its movements are so sluggish, that the quantity of oxygen which the fluid part of the blood conveys to them, is sufficient for their need. 236. Of the properties of the Liquor Sanguinis, whilst it is circulating in the vessels, the microscope tells us nothing; since it constantly remains in the state of a transparent fluid. But if the blood be withdrawn from the living body, it soon undergoes a very curious and important change. A large portion of it passes into the solid state, forming the crassamentum or clot; whilst there remains a transparent liquid of a yellowish hue, which is termed the serum. It is evident that the clot contains all the red particles; but it is easily proved that its coagulation is not due to them. For the blood of a Frog, or of any other animal having blood-discs sufficiently large, may be caused to pass through filtering-paper, which will retain and collect its blood-discs, allowing the liquor sanguinis to flow through it; and this fluid will coagulate just as completely as if these particles were retained in it. Again, in certain conditions of the blood (generally resulting from disease), even when the coagulation is allowed to take place in the ordinary manner, the fibrin and the red particles separate from one another, the latter gradually subsiding, whilst the former are left at the surface; and the upper part of the clot is then nearly colourless, exhibiting what is commonly known as the buffy coat or crust; whilst the lower part of it includes the red particles, and has a very deep colour. The buffy coat, being composed almost exclu LIQUOR SANGUINIS-COAGULATION. 211 sively of the fibrous network, is very firm in its texture, being sometimes almost leathery in its character; whilst the lower part of the clot, which is chiefly composed of the red particles, loosely bound together by scattered fibres, is very soft, and easily broken asunder. This effect may be also produced, by acting on healthy blood with certain substances which retard its coagulation, such as a strong solution of Glauber's salt; for if sufficient time is allowed, the red particles will subside in consequence of their greater specific gravity, leaving a colourless layer of fibrin above them.-It is of the liquor sanguinis, in a concentrated form, that those exudations consist, which are poured out from the blood for the repair of injuries, and which pass spontaneously into the condition of a simple form of tissue (§ 393). 237. When a very thin slice of the clot is examined with a microscope, it is found to be made up of a net-work of an imperfectly fibrous character, interlacing in every direction, and including the blood-discs in its meshes. These fibres are produced by the spontaneous change in the fibrin of the blood, from the fluid to the solid form. So long as the blood is circulating in the vessels of the living body, so long does its fibrin remain dissolved in the watery part of it; but so soon as it is withdrawn from these, and is allowed to remain at rest, it undergoes this remarkable change. If fresh-drawn blood be continually stirred with a stick or beaten with twigs, the fibrin coagulates in irregular strings, which adhere to the stick or twigs; and it does not then include the red particles, which are left behind in the fluid. In this manner it may be completely separated from the other elements of the blood, which have not in themselves the least tendency to coagulate spontaneously. Although forming a large proportion of the substance of the clot, the fibrin, when dried, does not consti tute more than from 2 to 3 parts by weight in 1000 of blood. This proportion is augmented to 6, 8, or even 10 parts, in severe inflammatory diseases. 238. When the fibrin and the red particles have both been separated from the blood, there remains a fluid, the serum. in which a good deal of albumen is dissolved, together win fatty matter, and other organic substances; with the addition of saline matter, of which a considerable proportion is chloride of sodium, or common salt. The proportion which the solid 212 SERUM-USES OF BLOOD. matter of the serum bears to the whole mass of blood, in health, is about 53 parts in 1000; and of these about 40 parts are albumen, 8 parts saline matter, and 5 parts fat, with certain ill-defined substances, of which some appear to be organic compounds that are undergoing metamorphosis into solid tissues, whilst others are the products of the decay of the tissues, which are being progressively withdrawn and eliminated by the excretory organs. 239. The influence of the Blood as a whole upon the animal as well as on the nutritive functions, is easily proved. When an animal is bled largely, it is gradually weakened as the flow proceeds, and at last it seems to lose all consciousness and power of movement. If allowed to remain in this condition, it seldom or never recovers of itself. But if we inject into its veins, by small quantities at a time, blood similar to that which it has lost, the apparent corpse becomes as it were reanimated, and all its functions are completely re-established. The importance of the red particles is manifestly seen in the effect of this remarkable operation, which is called the transfusion of blood; for if, instead of blood freshly obtained from another living animal, we inject serum without these particles, the effect is but little greater than if so much water were introduced, and the animal dies of the hæmorrhage. By this operation, practised on the Human subject, many valuable lives have been saved, that would otherwise have been destroyed by loss of blood. Again, if, by mechanical means, as by tying the principal blood-vessel going to any organ, we cause a permanent diminution to any considerable extent, in the quantity of blood with which it is supplied, a decrease in its size is soon apparent, and it may even shrink almost to nothing. On the other hand, we observe that, the more active the function of a part, the larger is the quantity of blood with which it is supplied. Thus, when the antlers of the Stag, which fall off every year, are being renewed, the arteries that supply the parts of the skull from which they spring, are greatly increased in size ; but they shrink again, as soon as the growth of the horns is completed for that year. A similar increase takes place among animals that suckle their young, in the size of the arteries that supply the mammary glands, by which the milk is formed; and these also shrink, when this liquid is no longer required. USES OF SEPARATE CONSTITUENTS OF BLOOD. 213 240. The following appear to be the chief uses of the principal constituents of the Blood, considered separately, in the general economy:- -The fibrin is the material which is most assimilated to the condition of the solid tissues, having the power of passing from the liquid state into a low and simple form of organization. It was formerly supposed to be the nutritive material at the expense of which the solid tissues generally are immediately produced; the muscular substance, in particular, being regarded as chemically identical with it. But there is now good reason to think that the greater part of the tissues form themselves at the expense of the albumen of the serum and perhaps of the globulin of the red corpuscles; and that the purpose of the fibrin is chiefly to give origin to those simple forms of fibrous or connective substance, the production of which is the first step in the reparation of injuries. Were it not for its power of coagulation, the slightest cut or scratch might become fatal, from the gradual draining-away of the blood; and such, in fact, has actually happened, in cases of disease in which the fibrin is deficient. The presence of fibrin also gives a degree of viscidity to the blood, which, as experiment proves, favours (instead of resisting, as might have been expected) its passage through capillary tubes; and thus, when there is a deficiency in this ingredient, local stagnations and obstructions in the circulation of the blood are very liable to occur. The albumen of the blood may be considered, like that of the egg, as the raw material, at the expense of which (in combination with fat) every other organic compound in the body is generated. It is, as we have seen, the substance to which all the tissueforming elements of the food are reduced in the process of digestion; and in this condition it seems to be continually appropriated by the acts of self-formation that are taking place, with varying rapidity, throughout the body, just as the albumen of the egg is appropriated by the self-formative operations of the embryo. There is strong reason to believe that a large proportion of the solid tissues regenerate themselves by the direct appropriation of this material; and if (as has been already stated to be probable) the simple fibrous tissues find their material in the fibrin, and the muscular substance in the globulin of the red corpuscles, it is from the albumen that these substances are themselves elaborated, both of them 214 USES OF SEPARATE CONSTITUENTS OF BLOOD, being, as it were, in process of organization. The albumen of the blood further serves to supply the albuminoid matters which are required as constituents of various secretions, especially those which are concerned in the digestive process, as the saliva, the gastric juice, and the pancreatic fluid. Α large amount is daily drawn-off for the production of the peculiar ferments contained in these secretions, whose action upon the food is necessary for its reduction to the form in which alone it can be received into the circulating current. Hence the making of new blood involves a considerable expenditure of the old. 241. The liquid in which the fibrin and albumen are dissolved, has a considerable power of absorbing gases; and this is greatly increased by the presence of the saline matters which it holds in solution. Hence the liquor sanguinis not only sustains the nutrition of the body, but can also serve, to a considerable extent, as a medium of communication between the lungs and the tissues. In this kind of activity, however, it is completely surpassed by the red corpuscles (§ 235). Independently of their use in ministering to the function of Respiration, there seems reason to believe that the red corpuscles are also subservient to that of Nutrition; for a certain conformity which exists between the organic and mineral substances they contain (§ 232), and the composition of Muscle and Nerve, taken in connexion with the manifest relation between their number and the activity of the Nervo-muscular apparatus, makes it probable that they have it for their especial office to prepare the materials which are to be used in its production and renewal of those tissues. The saline matter of the blood has many important offices: thus it furnishes the mineral ingredients which are requisite for the production of the tissues and secretions; it helps to preserve the organic substances from decomposition; and, in conjunction with the albumen, it keeps up the density of the serum to the point at which it is equivalent to that of the contents of the red corpuscles, without which balance the condition of the latter would be seriously impaired (§ 231). Finally, the fatty matters of the blood are subservient to two very important functions-the maintenance of heat, and the formation of tissue. They maintain the combustive process, whenever there is a deficiency of more readily combustible material; and they also take part with |