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

BOOK 1

CHAPTER 4

THE WEARING OUT OF THE MACHINE AND ITS REPRODUCTION

§ 1. Age and Decay.

wy. § 2. Reproduction and Fertilization. and Development of the Embryo. § 4. Rhythm and Birth.

§ 3. The Growth 8 5. Childhood,

Adolescence, and Maturity.

§ 1 Age and Decay

N the course of time Mr. Everyman

discovers that he is growing old. He has avoided complex chills, his leucocytes have defeated a score of dangerous infections, he has had wounds and they have healed, but nevertheless it is borne in upon him that he cannot go on for ever. He realizes a slow rusting up of this strange machine with which he is identified, and neither he nor any help he can call in seems able to arrest that inevitable march towards cessation.

The amount of specialized tissue that a body can lay down is limited. We possess, normally, the apparatus for making two sets of teeth, but no more. When any tooth of the second set is worn out, whether by decay or by inadequate blood-supply, or by sheer grinding away of its hard surface, it leaves a gap that can never be filled again. So with our softer tissues. Our nerves and muscles and glands, just like other delicate machines, are damaged a little by their own working, and we cannot keep them in repair for ever. For this reason after we have lived for some forty years our bodies begin to wear away; gradually, but with gathering speed, we lose weight.

This wearing-away chiefly involves the more specialized tissues. Skeletal or connective tissue, which consists of anatomically undifferentiated cells loafing about among a scaffolding that they have built up, is relatively unaffected. It is the brain and muscles, the kidneys and liver, that shrink. Moreover, different tissues dwindle away at different rates. The brain-tissue goes more rapidly than the cerebral arteries, so that the latter, having their original length but a shortened course, become twisted into curious loops and spirals. Similarly the skin does not shrink at all fast, so that, as the underlying muscles wear away and the fat stores are used up, it becomes thrown into wrinkles. As our specialized cells degenerate our faculties gradually become blunted.

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We cannot see or hear as acutely as before, our muscles are less powerful, our digestion is less efficient, our brains less active and less accurate.

The chemical basis of this wearing out is at present not understood. Old age seems to be associated in some way with defective calcium metabolism; the brittleness of senescent bones is due to the resorption of lime salts into the blood. Moreover, there seems to be an accumulation of poisonous substances in the blood. It has been shown that if a culture of healthy tissue from a growing chick (Chapter 2, § 2) be mounted in blood-serum from a very old bird the cells cease forthwith to divide and grow.

The slow exhaustion of old age may also be complicated by other changes; in the great majority of old people (but not in all) there is a gradual stiffening (sclerosis) of the arteries, and there may be a similar stiffening in other tissues.

Sooner or later one or other of the essential organs fails and the body dies. In most cases of natural death in people over fifty the failure occurs in the circulatory system; the heart stops, or one of the stiffened arteries bursts, and all the other tissues perish because of the cessation of their blood-supply. In other cases the failure occurs elsewhere—in the braincentre that calls forth the respiratory movements, for example.

It is important to realize that our cells do not die because mortality is inherent in their internal structure. They die because they are parts of a very complicated system based on co-operation, and sooner or later one of the tissues lets the other down. In death from heart-failure, for example, it is wrong to suppose that the heart’s muscle-fibres die and therefore stop beating ; their stoppage precedes and causes their death. ‘The cells first weary of their unending labour and stop beating, and only subsequently die, like all the other cells of the body, because, as a result of their own stoppage, their bloodsupply is cut off. As a matter of fact,