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

THE COMPLEX BODY-MACHINE AND HOW IT

each square millimetre (that is, about double the cross area of an ordinary pin) contains well over a thousand, and all the capillary vessels in a man’s muscles put end to end to form a continuous tube would girdle the earth two and a half times. When filtering through this network of fine tubes, the amount of exposed surface of the blood is enormous; a single cubic centimeire of blood will have about ten thousand square centimetres of surface at which chemical interchanges can take place. So we come to what we have already seen in the frog’s foot.

Deoxygenated blood flows from the capillaries into the veins slowly and at low pressure. The driving force of the heart is mainly spent. ‘The flow in the veins is helped by various accessory factors. ‘The most important of these factors is a series of valves, baggy membranous projections of the veins’ walls into their cavities, which only allow blood to pass them in one direction. These are present even in the smallest endbranches of the veins. The veins are squeezed by the muscular movements of the body, which thus assist in driving blood along them. During inspiration, for example, the veins in the abdomen are pressed upon as the diaphragm descends, and during voluntary movement of the limbs blood is driven out of the capillaries and veins of the contracting muscles. It is also possible that the muscle-fibres in the walls of the veins themselves are capable of slow rhythmic contraction. Because of the valves, the various factors tending to compression of the veins can drive blood in one direction onlytowards the heart.

The internal pressure in the veins being much lower than that in the arteries, and there being no violent pulse to resist, the walls of the veins, although they include muscular and elastic elements, are not nearly as thick nor as strong as those of the arteries. The blood-flow in the veins, deriving its principal motive force, not from a special pump but from the incidental action of unrelated organs, is very much more sluggish than that in the arteries. The cross-section of any vein is about double that of the corresponding artery, and inside it the speed of the blood is only about half as great.

Fig. 17. A vein cut

open, Showing two pocket-shaped valves.

WORKS

The time taken by any particle of blood to round the whole double cycle varies, of course, according to the particular artery along which it happens to be driven; a journey from the heart to the muscles of the ribs is very much quicker than one to the toes. On the average, however, in a man, the time taken is slightly less than a munute.

So it is that the blood travels about our bodies. However, there is another system of vessels which play a part in the circulation, but about which nothing has hitherto been said.

Let us consider once again that allimportant part of the circulation—the capillary network. We have spoken as if the capillary blood-vessels were in immediate contact with the surfaces of the cells, but this is not strictly true. The capillary vessels are, roughly speaking, cylinders ; the cells, according to the nature of the particular tissue, may be cylindrical, cubical, spindleshaped, or may assume various other forms. In between the cells and the capillaries there are minute irregular spaces, and these spaces are filled by a clear plasma-like fluid—the lymph. In the interchange of substances between blood and tissue, lymph plays the part of a go-between ; substances diffuse from the blood and flesh into the lymph, and from the lymph into the blood and flesh. Lymph has been compared to a middle-man, taking substances from the blood and handing them to the cells, and vice versa ; but, unlike a human middleman, it extorts no profit from this turn- Fig. 18. How the valves prevent over, and blood from flowing in the wrong as a matter direction. of fact it plays no active part therein. Lymph, in the intercellular spaces, is simply a fluid through which substances diffuse. It comes from the blood and it is destined to return to the blood. The capillary walls, as we have seen, are very thin, and the blood in the capillaries is under pressure. The pressure is not great, but so delicate are the walls that it is high enough to drive fluid through them, and there is always a very slow, very steady seeping of blood-plasma from the capillaries