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

BOOK 1

CHAPTER 3

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

§ 1. A Study of Adjustment. § 2. Chemical Messengers. § 3. Man and Mouse as Indwiduals. § 4. The Controlling System. § 5. Sensation and the Senses. § 6. The Nervous Mechanism and the Brain.

§ 1 A Study of Adjustment

E are “fearfully and wonderfully

made,” says Holy Writ, and all that has gone before must seem to the reader but an elaboration and filling in of that statement. Probably the thing that will seem most marvellous to him will be the co-ordination of this intricate diversity of cells and organs, all so capable of a measure of independence and all so disciplined towards the common end of being Mr. Everyman and obeying his will. We have already noted that the co-ordination, amazing as it is, is not perfect. We shall be better able to grasp the difficulties in the way of perfection and the extraordinary nature of the adjustments that do occur, if we make a brief study of a particular set of these. We will choose the heart and lungs under varying conditions of exertion and repose for the study, and we think the facts we shall state are sufficient to astonish anyone not already hardened to them by familiarity.

Now the first adjustment that we have to consider is the adequate supply of oxygen to the working cells of the body. In a resting man the heart, the stomach, the kidneys, and the respiratory muscles are active, and their oxygen demands must be satisfied ; but the great mass of his muscles —the voluntary muscles—are quiescent, and their oxygen-requirement, although it is not zero, is very small. During violent exercise, on the other hand, the oxygen-need of the voluntary muscles is enormous. A resting man consumes roughly 250 cubic centimetres of oxygen per minute (just under half a pint), but during exercise he may use ten or fifteen times this amount. These are the opposite extremes of his requirements. In a sudden crisis he may have to flash from one to the other in a minute or so. Even during ordinary life, the oxygen-requirement varies widely, walking up a staircase requires very much more energy than

walking at the same speed on the level. To meet this constant variation in the amount of muscular exertion, there have to be corresponding changes in the breathing rhythm and in the blood-flow. When activity increases, the blood-flow must be accelerated, so that oxygen can be brought to the tissues more rapidly, and the rate at which air is breathed must also be augmented so that more oxygen can get into the blood. Let us take first the regulation of the breathing. The contraction of the muscles concerned in breathing—the intercostal muscles between the ribs, and the muscular part of the diaphragm—is due to nervous impulses coming from the brain. The impulses emanate from a centre at the lower end of the brain-stem, and travel to the muscles concerned along certain nerves—the intercostal nerves to the intercostal muscles and the phrenic nerves to the diaphragm. It is a continual series of rhythmic nervous discharges from this centre that keeps the respiratory system in motion.

Now the activity of the respiratory centre can be modified, either by nervous impulses from other sources or by changes in chemical composition of the blood which bathes it. Of these factors the latter is probably the more important in regulating breathing to correspond with exercise taken. The respiratory centre is very sensitive to the amount of carbon dioxide in the blood, responding to any increase by a more vigorous and rapid stream of impulses. If the carbon dioxide in a man’s blood is increased by as little as 3 per cent. of its normal value, the rate of breathing is doubled. Similarly if the carbon dioxide in blood is unusually low the rate of breathing falls off. Jf it as decreased by only 3 per cent. his breathing stops altogether. On the other hand, the centre is not very sensitive to the amount of oxygen in the blood. Considerable oxygen-lack does produce compensatory effects, but these effects are feeble compared with those of the

_smallest change in the carbon dioxide.