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

THE GCOMPLEX BODY-MACHINE AND HOW IT WORKS

of non-living fibres of great strength and elasticity that they have laid down. ‘Tendons and ligaments are much the same sort of thing, but more so—with a greater proportion of fibres, and smaller and fewer cells. The connective-tissue cells are carpenters, and build the non-living scaffolding that supports the body; but the parallel would be closer if the scaffolding made by human carpenters consisted of their consolidated sweat. Finally, blood consists of cells floating in a non-living watery fluid, and the watery part sustains most of the transport duties of blood.

§ 3 Blood

“The life which is the blood thereof,’ says the Bible, and we may best go on to our examination of the life in a man’s body by considering the nature and functions of the blood. Where the blood is not circulating the processes of life, in aman or mouse, cease almost immediately. Stop the blood-supply to the head and in a moment the brain is vacuous and the body it is directing stumbles and faints. Lungs, liver, kidneys and stomach, all our internal organs, are mere factories and depots to purify and feed this essential vitalizing stream. An almost infinitesimal change in its composition makes us exalted or depressed or fills us with unwonted fears or desires. Manifestly our next step on our way to the understanding of this vast cell-community, an animal body, must be a study of the blood.

It should be realized at once that blood is not a simple lifeless fluid like water or milk ; it is, in biological language, an active living “ tissue.” Nearly half of its volume consists: of living cells. The cells of blood float in a watery medium, and blood as a whole is therefore a fluid, but it is nevertheless quite as much a tissue as cartilage or bone—it is simply a liquid tissue.

The cells of blood are of two kinds, red and white, and it is to the presence of vast quantities of the former kind that the red colour of blood is due. A human red blood cell is a disc about seven-thousandths of a millimetre across, about one quarter as thick at the rim, and rather thinner in the middle, or, putting it into English measures, 3,500 red blood corpuscles side by side would measure about an inch. It has no nucleus. Inside the cell there is an apparently homogeneous fluid in which no structural elements can be discerned, containing as one of its chemical constituents

a red pigment known as haemoglobin. The whole is bounded by a membrane, so delicate that it cannot be seen even with the highest powers of the microscope, and which is believed to consist of a double layer of fatty molecules arranged side by side in a regular way. ‘The white blood cells are of several different kinds, but they are all alike in being a little larger than the red cells, in having conspicuous nuclei, and, like isolated cells of a tissue-culture, capable of a slow change of shape. The cells are present in enormous numbers; a cubic millimetre of blood contains about ten thousand white and six million red—that is to say, there are more red cells in one drop of blood than there are people in the United Kingdom.

A few cells from human blood. Below,

Fig. 11.

Above, some red cells are seen in various positions. three kinds of white cells.

The reader contains just about a gallon of blood, so imagine the vastness of this continually circulating population. ‘The red cells are concerned with the transport of oxygen, and the white cells include among their many duties that of combating bacterial parasites.

The red and white cells, we repeat, constitute one-third of the volume of blood, the remaining two-thirds is water, carrying a great and varied assortment of chemical compounds. This water with its dissolved substances is called the plasma; it forms a clear, nearly colourless fluid in which the blood-cells float. Plasma is the non-living part of blood, but it is nevertheless by no means the least important, for it is as