The science of life : fully illustrated in tone and line and including many diagrams

THE GOMPLEX BODY-MACHINE AND HOW IT WORKS

indefinitely once our bodies were punctured. Some unfortunate people have blood that does not clot. In the hereditary condition known as Haemophilia, in which the blood is incapable of clotting, patients lose large quantities of blood from even trivial cuts, and have been known to bleed to death in the dentist’s chair. Normal clotting, which fills the wound and so checks the escape of blood, involves as an essential feature the conversion of a certain substance dissolved in the plasma, fibrinogen, into fibrin, which is insoluble and which precipitates therefore to form the fibrous clot. This conversion is the result of an extraordinarily complex chain of preliminary reactions, in which a number of different substances participate, some of which exist ready-made in the plasma, while others are contributed by damaged cells at the point of injury. The plasma carries this fibrinogen and various accessory substances, so that this protective process of clotting takes place when occasion demands.

And still there is more to tell of this marvellous fluid, which is, as the Bible has it, the ‘‘ life” of a higher animal. It feeds, it comforts, it protects. It is also a means of communication between part and part. Itcarries messages that secure the harmonious co-operation of one organ with another.

Twenty or thirty years ago it was thought that the co-operation of part with part was ensured through the nervous system alone, either consciously through the brain or unconsciously by the subordinate systems of nervous communication through the spinal cord and inferior centres. But nowadays we are beginning to realize that a very large part of the harmonizing task is done through substances emitted by one organ and reaching another by way of the blood. The canal system is more important relatively to the telegraphic system than was once suspected. Concerning these substances, the ‘‘ internal secretions,’ as they are called, we shall give some interesting details later. Here we note them simply to round off our account of this essential fluid which bears our lives along. One more fact about it, however, we may add before we close the section. Some constituents of the blood seem to be of no use whatever ; they happen to be unavoidable. Such, for example, is the dissolved nitrogen which the

plasma picks up in the lungs when it is taking up oxygen for the cells. The nitrogen has a free ride round the system, but apparently has no function whatever.

Such is the nature and composition of the blood. About one-twentieth of the weight of a normal man is blood, and in the meshes of a network of blood-streams, all the life in our bodies goes on. We will next take a glance at these streams and note how they are driven round the body.

8 4 The Course of the Blood

Tt is not difficult to see the blood actually The delicate

at work among the tissue-cells.

FR if A ed

A fine artery branching out into capillaries, as Seen in the frog’s foot.

web of a frog’s foot, for example, can be examined under a microscope, without being severed from the rest of the frog or indeed causing any pain, and we can observe the vital business in progress. The field of vision is occupied by masses of greyish tissue, in which the boundaries of individual cells can be made out only with difficulty. In the frog’s foot pigment-cells can be seen dotted over the field—black star-shaped masses with irregular processes radiating out from a common centre. Riddling the sheet of tissue under examination, and apparently dividing it into lobes, is a network of canals, the blood-vessels. In these vessels the course of the blood may

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