The blood lies everywhere in closed tubes, and consequently does not come into direct contact with any of the cells which make up the body, except those which float in it and those which line the interior of the bloodvessels. At two parts of its course, however, the vessels through which it passes have extremely thin walls, and through the walls of these capillaries liquid transudes and bathes the various tissues. The transuded liquid is called lymph; the blood makes lymph, and the lymph directly nourishes all the tissues except those mentioned above, with which the blood itself comes in contact.

Dialysis

When two specimens of water containing different matters in solution are separated from one another by a moist animal membrane, an interchange of material will take place under certain conditions. If A be a vessel (Fig. 56), completely divided vertically by such a membrane, and a solution of common salt in water be placed on the side b, and a solution of sugar in water on the side c, it will be found after a time that some salt has got into c and some sugar into b, although there are no visible pores in the partition. Such an interchange is said to be due to dialysis or osmosis, and if the process were " allowed to go on for some hours the same proportions of salt and sugar would be found in the solutions on each side of the dividing membrane.

How much blood is there in an average sized man?

Why does the blood not directly bathe most of the tissues? What cells come in contact with it? What are the capillaries? What is lymph? What is the nutritive function of lymph ?

What happens when watery solutions of different substances are separated by a moist animal membrane? Illustrate. What is such an interchange called? What would be the result at the end of some hours?

The Renewal Of The Lymph

Osmotic processes play a great part in the nutritive processes of the body. The lymph present in any organ gives up things to the cells there and get things from them; and so, although it may have originally been tolerably like the liquid part or plasma of the blood, it soon acquires a different chemical composition. Dialysis then commences between the lymph outside and the blood inside the capillaries, and the latter gives up to the lymph new materials in place of those which it has lost, and takes from it the waste products which it has received from the tissues. When this blood, thus altered by exchanges with the lymph, reaches again the stomach and intestines, having lost some food materials, it is poorer in these than the richly supplied lymph around their cells, and takes up a supply by dialysis from it. When it reaches the excretory organs it has previously picked up a quantity of waste matters, and loses these by dialysis to the lymph there present, which is specially poor in such matters, since the excretory organs constantly deprive it of them. In consequence of the different wants and wastes of various cells, and of the same cells at different times, the lymph must vary considerably in composition in various organs of the body, and the blood flowing through them will in consequence get and lose different things in different places. But, receiving in its passage through one region what it loses in another, its average composition is kept pretty constant;and, through interchange with it, the average composition of the lymph also.

How does the lymph in an organ come to differ chemically from the blood plasma which supplied it? What results? What happens when the blood thus changed reaches stomach or intestine ? What when it reaches excretory organs ? Why does the lymph vary in composition in different parts of the body ? How does this affect the blood ?

The Lymphatic Vessels Or Absorbents

The blood, on the whole, loses more liquid to the lymph through the capillary walls than it receives back at the same time. This depends mainly on the fact that the pressure on the blood inside the vessels is greater than that on the lymph outside, and so a certain amount of filtration of liquid from within out occurs through the vascular wall, in addition to the dialysis. The excess is collected from the various organs of the body into a set of lymphatic vessels, which carry it directly back into some of the larger blood-vessels near where these empty into the heart; and as fast as this onward flow of the lymph occurs under pressure from behind, it is renewed in the different organs, fresh liquid filtering through the capillaries to take its place as fast as the old is drained off.

Since the lymphatic vessels may be said to take up or absorb the excess of liquid drained from the blood and also the effete matters of the various organs, they are frequently called the absorbents.

Lacteals we have already learned to be only another name for the absorbents of the small intestine (p. 165).

How is the average composition of the blood maintained? How that of the lymph?

Give another name for the lymphatic vessels. Does the blood on the whole gain or lose liquid to the lymph as it flows through the capillaries ? Explain why. What becomes of the excess of liquid drained off from the blood ?

Where do the lymphatic vessels convey it? What produces the onward flow of lymph? How is the lymph thus drained off replaced? Why are the lymphatics called absorbents?

What is meant by the lacteals?

Histology Of Lymph

Pure lymph is a colorless, watery looking liquid; examined with a microscope it is seen to contain numerous pale corpuscles exactly like those of the blood. It contains none of the red corpuscles.

Chemistry Of Lymph

Lymph is not quite so heavy as blood, though heavier than water. It may be described as blood minus its red corpuscles and considerably diluted, but of course in various parts of the body it will contain minute quantities of substances derived from neighboring tissues.

Summary

To sum up: the blood and lymph provide a liquid in which the tissues of the body live; the lymph is derived from the blood, and affords the immediate nourishment of the great majority of the living cells of the body; the excess of it is finally returned to the blood, which indirectly nourishes the cells by keeping up the stock of lymph. The lymph itself moves but slowly, but it is constantly renovated by interchanges with the blood, which is kept in rapid movement by the heart, and which, besides containing a store of new food-matters for the lymph, absorbs the wastes which the various cells nave poured into the latter.

What does lymph look like? What is seen when it is examined with a microscope?

How does lymph differ in density from blood and from water? How may it be briefly described? What does it contain in various regions of the body?

What do the blood and lymph provide? Whence is the lymph derived? What does it afford? What becomes of its excess? How does the blood play a part in nourishing the cells of the body? Which moves faster, lymph or blood? How is the lymph renovated? What keeps the blood in motion? What does the blood do besides renewing the food-matters in the lymph?