The Functions of the Blood are to carry nourishment to every tissue and organ of the body. It may be compared to a stream flowing through a tract of country, and giving off branches in all directions, so that it waters every quarter of the district. We know that that tract of country is likely to be extremely fertile, and we know what a change would come over it if the river were dried up; or, if one branch only were suddenly cut off, we know what a contrast the part formerly supplied by that branch_ would present to the rest, still watered as before. Now for the tract of country substitute the human body, and for the stream, with its many branches to every part of the country, substitute the blood carried in channels (the blood-vessels) to every portion of the body, and we have a very striking similarity. For the blood flowing through a part of the body as surely bathes the tissues beside which it flows as does a river water the fields along its banks. Thus the liver cells (p. 200) are situ ated in groups on the blood channels, which exist in such abundance in the liver. As the stream of blood flows past, they select from it what they need for their work, just as different trees and plants along the bank of a stream will suck up what nourishment they need. The liver cells select from the blood the substances out of which they may manufacture the bile salts, the bile colouring matters, &c. It is the same blood that flows through the brain, and here it comes in contact with brain cells, whose function, connected with thinking, feeling, will ing, &c., is very different from that of the liver cells. Nevertheless from the same blood they find nourishment for their life and the source of their activities. They too select from it the substances they need, and convert them into that which it is their business to produce, and which is something very different from bile salts and colouring matters. It is the same blood that flows through a muscle and bathes the muscular fibres. They also find in it the elements that are necessary for restoring exhausted muscle fibres and building up new ones. It is the same blood that flows through the tissues of the eye and maintains in health and power this most wonderful of optical instruments. In short, just as in one garden you may have the lily and the rose near neighbours to strawberry beds and apple-trees, growing from the same earth and having the same rain and sunshine from heaven, yet each one selecting from the common nour ishment the elements it needs, and converting them into flower and fruit very different from one another, so the blood in the human body contains nourishment for the liver cell and brain cell, muscular fibre and sentient organ, which each selects as its needs dictate.
While this is the main function of the blood, it serves that other purpose of carrying away, in its current, from the tissues substances, pro duced by their work, whose removal is neces sary for the continuance of the healthy life of the part.
It is, then, quite clear how conditions of the blood speedily affect the whole body. Suppose the blood to be insufficient in quantity. Each tissue and organ gets a supply, but not enough. Its vigour is, therefore, diminished and its effi ciency impaired. If the blood be equally dis tributed, all parts of the body may suffer equally and general symptoms of ill-health, not symp toms pointing out definitely one sufferleg part, are the result. In some cases one organ may suffer more than another, may get less than its own share of the diminished supply, and along side of the general symptoms, which point out the general condition, are others indicating some organ on which the privation is specially hard. The blood may, however, be sufficient in quantity but bad in quality. Some particular element may be wanting. For example, scurvy is held to be due to absence from the food of some element, perhaps potash salts, perhaps citric acid, which fresh vegetables supply. Sonic substance may be present that ought not. For example, the liver may have failed to separate bile, and substances are left in the blood which, being carried through the body, act directly or indirectly as a poison. Some material may also gain entrance by the food, or air, or in other ways, which vitiates the quality of the blood and impairs its value as a nourishing fluid. How dependent, therefore, the whole body is on the quantity and quality of the blood is evi dent, and will become more clear when the results of its impaired efficiency are studied in the succeeding section.