Our lesson on the blood showed us that fluid coursing through the body in tubes or pipes, which are called blood-vessels. We shall now proceed to examine these vessels, beginning with the veins, because we know something of them already. They are the vessels which can be seen as dark purplish cords just beneath the skin; they convey the blood back to the heart. We may easily trace them upwards from the hand, as they unite into larger and larger trunks. We might compare them to the numerous tributaries of a great river, all bringing their contributions to swell the main stream.
In all these vessels the blood is perpetually flowing one way-from the smaller into the larger trunks, and so on to the heart. We cannot see the blood flow along our veins, but a microscope shows it very clearly in the thin transparent web between the toes of a frog.
The veins, as seen just beneath the skin, are of a dark purple color. Theblood in them is not bright red blood, such as we usually see, but purple-almost black. It is, in fact, loaded with impurities-the waste, worn-out, dead particles of muscle, nerve, brain, and bone, which the blood in its course has caught up, and is carrying away to the heart. These waste matters could not be allowed to remain in the body. They would be as injurious there as a heap of rotting, putrefying refuse matter would be in and about our homes. Their very presence in the blood has changed its character and appearance; it was bright red blood once, now it is darkpurple and impure. It is the duty of the veins to collect up all this impure blood, and pass it along through larger and larger trunks, and finally discharge it into the heart.
Let us turn next to the arteries-the vessels which carry blood away from the heart. Their great main trunk, which springs directly from that organ, is a pipe rather larger than one"s thumb. This sends off branch after branch, each branch dividing again and again into smallerand still smaller ones, until they are at last as fine as the finest veins.
But the blood which passes from the heart along these channels is good blood, free from impurities, and loaded with materials for renewing those parts of the body which have been worn out. Its color, too, is changed to a bright red.
You would form a very good notion of these arteries if you could picture to yourselves the water-supply systemof some large town. Trace, in your mind, the course of the water from the central pumping station, first along the great main, then into the pipes leading from it under the roadways, then again into smaller and smaller branching pipes leading to the streets and lanes, and finally into the delivery pipes in connection with the individual houses.
The veins and their work might be as clearly illustrated by comparing them to the drainage system of the same town. The water company"s pipes bring pure, fresh, wholesome water to the houses, but this same water, after performing its office, finds its way, polluted with filth and all sorts of impurities, to the drain. The drains from the various houses run into larger pipes, and these into still larger ones, until the great main sewers are reached, and these carry the filthy stream away from the town, where it can do no harm. The arteries are usually embedded deep in the flesh, and cannot be seen. This is a most providential arrangement, because if an artery be cut or wounded, it is a very difficult matter to close up the wound and stop the bleeding. The person is in extreme danger of bleeding to death. Packed away deep in the flesh as these vessels are, they are comparatively safe from injury.
If an artery were removed from the body of any animal and examined, it would be found to be a plain, simple pipe, through which water might be poured from either end. If a vein were removed, and treated similarly, weshould find an important difference in this respect. It would be easy enough to pour water through from one end, but if we tried to do so from the opposite end, the pipe would become closed or stopped up. The reason for the stoppage would soon become apparent if we slit the pipe open. The walls of the veins areprovided with little bags here and there, which stretch across the tube with their mouths opening all in one direction- towards the heart. These little bags, or pouches, form valves. They allow the bloodto flow readily past them on its way to the heart, but should it, from any cause, attempt to flow backwards, it would fill up the little pockets, and swell them out till they blocked the way altogether. There are no valves in the arteries.
We now clearly understand that in every part of the body there must he two sets of vessels-arteries to bring pure fresh blood, veins to carry it back to the heart contaminated with impurities. Every artery ends and every vein begins in a network of still smaller hair-like tubes- the capillaries. So close are these tiny vessels placed that it is impossible to prick the skin anywhere without piercing the walls of some of them, and causing the blood to flow.
The walls of the arteries are stout and muscular; those of the veins are thinner and somewhat flabby, but the capillaries have the thinnest of thin walls.
It can only intensify your feelings of wonder and surprise when you learn that in the horse the complete round of the circulation from the heart, along the arteries, through the capillaries, and back by the veins to the heart again, is accomplished in about half a minute.
