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

BOOK 1

to starve). Nevertheless, if proper precautions were taken, the limb could be kept alive by perfusing it along the arteries with blood. ‘This sort of thing is constantly done in the laboratory with detached nerves or muscles or hearts or stomachs or kidneys, because by using such methods it is possible to study living tissues without causing pain to living organisms.

On one occasion (a long time ago) a physiologist obtained the heart of a criminal aged thirty-five. He removed the heart eleven hours after the execution, and ran a suitable artificial fluid into it through the veins, to take the place of blood ; the heart revived and began to beat again, and was

Fig. 10. the cells

then studied and experimented upon for a full three hours. The heart of a coldblooded animal, such as a tortoise, will live for weeks under such conditions. Similarly the mammalian kidney can be removed altogether and supplied with artificial blood through a glass tube tied into its artery ; it will continue for hours to remove any undesirable matter from the fluid that runs through it. And so with other organs. ‘The indivisibility of the human body is then a spurious indivisibility, resulting from the fact that its cells are specialized, that the cells of a leg, for example, have come to rely on the cells of the heart for a supply of nourishing blood. On the other hand the indivisibility of a cell is absolute, for the

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THE SCIENCE OF LIFE

A slip of gristle, highly magnified, showing embedded in a hard substance that they themselves have made.

CHAPTER 2

cell represents the simplest possible way in which animal tissue can be organized ; without its essential parts the exchanges of energy, the complex interplay of chemical reactions that underlie life, cannot continue. It is in fact a primary indivisible unit of life.

‘The number of cells in the reader’s body is staggering. In the blood of an average man there are over fifteen million million cells in the blood alone ; his brain system contains nearly two thousand million ; and the total number in the human body is over 1,000,000,000,000,000—a thousand _ billions (and English billions, not American ones). They serve the body community in various ways and have various appropriately specialized forms. Some are of service because they can actively change their shape—such as muscle-cells ; others, the nerve-cells, are drawn out into enormously long, thin threads, and are like living telephone wires; others, more cubical, serve by exuding special chemical substances—such as the cells of the salivary or thyroid glands. We need not catalogue all the possible varieties, but can content ourselves with stating that there are well over fifty distinct kinds of cells to be found in every man’s body. But, since we are dealing with the ultimate nature of our tissues, it is interesting to note in passing that there are very important parts of our bodies that are not in any sense alive. Our cells are all alive, but we are not all cells.

Figure 10 shows the structure of gristle, or, as the biologist terms it, hyaline cartilage. The cells do not lie in contact with others but are separated by a stiff transparent substance. If a fragment of living cartilage be cultured, those cells that can escape will migrate out. into the serum, but the matrix between the cells will not move or grow. It is not alive, but an exudation made by the living cartilage cell in much the same way as spittle is made by the cells of the salivary glands. Nevertheless it is to this non-living substance that cartilage owes its stiffness and smoothness, the qualities that make it useful to the organism. Similarly with the other kinds of framework tissue. Bone consists of small cells imbedded in a mairix which has been made rigid by the deposition in it of lime salts, and it is upon this matrix, exuded (or, in biological language, secreted) by the bone-cells, that the strength of a bone depends. The binding connective tissue that permeates organs and holds them together consists of cells surrounded by a web