I explained in the previous chapter that the blood of the vertebrates may well be called hæmolymph, since in them the colourless, slightly opalescent fluid called "lymph" is continually poured through certain openings into the red blood, and mixed with it. In the earth-worm and other lower animals the red-coloured blood, or its equivalent—the "hæma," as distinguished from the "lymph"—is held in a closed system of vessels, and does not receive any of the lymph. When examined with the microscope, the blood, or hæmolymph, of man is found to consist of an albuminous, slightly sticky liquid, in which float an immense number of "corpuscles"—minute bodies, some rounded, some irregular, some bun-like, and some spherical. The most abundant of these are the "red corpuscles," of the shape of buns, slightly depressed on each surface. Three thousand two hundred of them could be placed lying flat side by side along the space of a measured inch. They appear pale greenish-yellow in colour under the microscope, but in quantity, lying one over the other, they allow only red and some blue light to pass through them, and so have a fine red colour. They consist of a small quantity of albuminous matter and water, and of a large proportion of a red-coloured, crystallizable, chemical substance dissolved in them, called hæmoglobin, or blood-red. It is this hæmoglobin which performs one of the most important duties of the blood, since it combines with the oxygen of the inspired air when the corpuscles are flowing through the fine vessels of the lungs, and carries it to the tissues in every part of the body, which greedily take the oxygen from the red corpuscles.
The red corpuscles of man's blood and that of the hairy suckling animals—the mammals—are not nucleated cells, but are regularly formed and renewed as they daily wear out, as fragments of larger mother-cells, which break up into these corpuscles, in the marrow of the bones, and some other situations where they are found. In all other vertebrates the red blood corpuscles have a kernel, or dense nucleus, and are complete "cells," usually oval, smooth and flattened in shape—a curious difference not easily accounted for. There are in a pint of the blood of an average man about two billions of these red corpuscles, and the amount of blood in the body is about one-twentieth of the total weight of the body—say, in a man weighing 160 lb., about 8 lb. or pints of blood. The clear, colourless lymph existing in all the lymph spaces of the body is probably about twelve pints. In many animals the red corpuscles are much less numerous than in man; for instance, a drop of human blood contains a thousand times as many red corpuscles as does an equal-sized drop of frog's blood. It is true that the frog's red corpuscles are a good deal bigger than those of man, but the result is that the human blood is some hundreds of times richer in hæmoglobin than the frog's, and has a proportionately greater power of carrying oxygen from the lungs to the tissues, and keeping up the slow, burning process, or oxidation, upon which the activity of the body, as well as its warmth, depend. The body depends upon its supply of oxygen as a steam-engine depends upon the oxygen of the air, which keeps its coal-fire burning.
The pace of the blood-stream which is produced by the force-pump action of the contractions or beats of the heart is tremendous. It courses along at the rate of ten inches in a second in the big arteries and veins, and it has been carefully ascertained by experiment that a heartful of blood (which in a big man is about half a pint for each half or "side" of the heart)—or let us speak of a single corpuscle—is driven out of the heart through the great artery or aorta to the most remote parts of the body, and is back again at the heart, after running through endless branches of arteries, smallest capillaries, and thence into fine veins, bigger veins, and the biggest vein, in twenty to thirty seconds, the time occupied by twenty-five to thirty heart-beats. The walls of the arteries are firm, though elastic, and it is no wonder, with this tremendous pressure and pace on the liquid within, that when an artery is cut the blood spurts out to a distance of several feet.
The colourless liquid of the blood contains, besides the red corpuscles floating in it, others brought to it in the lymph and derived from various connective-tissue spaces and special nodules or "glands." They are outnumbered by the red corpuscles in the proportion of five hundred to one. They are colourless, and bigger than the red corpuscles. Most of them continually change their shape, and consist of active, moving protoplasm. These are the "phagocytes," which, besides acting chemically upon the constituents of the blood-liquid, take into their substance (as does the amœba or proteus-animalcule) and digest and destroy all foreign or dead particles, and the bacteria which may find their way into it. They pass out, forcing their way through the excessively thin walls of the finest capillaries—blood-vessels not wide enough to admit two of them side by side—and enter, to the number of thousands, the tissues which have been wounded or poisoned by bacteria, to carry on their all-important protective "scavenger" or "police-constable" work.
Inflammation is the slowing of the blood-stream by dilatation of the vessels at an injured spot, in order to allow the phagocytes to make their way out of the blood-stream into the tissues, and so get to close quarters with the enemy. There are other excessively minute dustlike particles called "platelets," which are sometimes very abundant in the liquid of the blood. Besides the duties of oxygen-carrying and scavengering the blood has other great and vitally important business. It has to distribute nutriment, to pick up waste oxidized chemical products and get rid of them, and to distribute and equalize the heat which it carries around the body like a perfect hot-water warming installation.
CHAPTER XXXV
FISH AND FAST DAYS
MOST people are familiar with the fact that fasting in the Christian Church has from early times been of two degrees—one in which no flesh of beast or bird or fish, not even eggs, not even milk, may be consumed, and a less severe degree in which the eating of fish is allowed. It is not at first sight clear why the eating of fish—and even of birds such as the Barnacle goose and the Sooty duck, supposed to be produced from fish—has been permitted by the Christian Church, since the flesh of fish is highly nourishing and an excellent substitute for the meat of beasts and birds, and a man fed upon it is far from suffering the effects of true "fasting." Many races and out-of-the-way people live entirely upon vegetables and a little fish, and do very well on that diet.
It has been proved by some learned inquirers that there was a special significance about the permission by the early Christians of a fish diet during so-called "fasting." Real and complete fasting, abstention from all food, for a day or even a week, was and still is practised by some Eastern peoples as a religious exercise. It is a matter of fact that an ecstatic condition of mind is favoured by complete fasting, and conditions favourable to illusions of various kinds are so produced. But the later Christians seem to have regarded the partial fasting during Lent and on certain days of the week as a sort of protest against gluttony and excess, and there is no objection to it among Protestant Churches excepting that it must not be claimed as a merit or the equivalent of "good works."