As these two fluids are generally mixed in the thoracic duct, whence the lymph is commonly obtained for examination, we may discuss them together, though the lymph might more properly be considered with the distribution of the nutrient materials to the tissues.

The fluids coming from the tissue drainage, from the lymphatic glands and from the lacteals of the alimentary tract, when mingled in the thoracic duct, form an opaque mixture which holds a considerable quantity of proteid in solution, and contains a number of morphological elements, viz.: (i), protoplasmic cells similar to those found in the lymph follicles, and in most essential points identical with the pale cells found in the blood; (2), some red blood corpuscles which gave the fluid in the thoracic duct a pinkish color; (3), a quantity of very finely divided fat, which varies in proportion to the amount of fat recently digested; (4), other minute particles of unknown function and origin.

When removed from the body and allowed to stand, the lymph becomes converted into a soft jelly. This coagulation, no doubt, depends upon the chemical changes in the lymph which give rise to fibrin, the formation of which will be discussed more fully in a future chapter. The amount of fibrin formed in the lymph is very small, and, therefore, the clot is very soft, and shrinks considerably.

The lymph of the thoracic duct contains three forms of proteid: (1), serum albumin, which can be coagulated by heat; (2), alkali albumin precipitated by neutralization; and (3), globulin. It also contains soap in solution, cholesterin, grape sugar, urea, leucin, and some salts, particularly sodium chloride, and the sulphates and phosphates of the alkalies.

The quantity of chyle which can be obtained from the lacteals is small, and, therefore, its thorough investigation is difficult. The fluid from the lacteals differs from the mixed lymph in appearance and constitution only during digestion, and then chiefly in containing a greater amount of fat and solids derived from the intestinal cavity.

On their way to enter into the blood current both the lymph and chyle undergo changes. Before passing through the lymphatic glands the fluid contains much fewer lymph corpuscles than after it has traversed the glands: from this fact, and from the structure of the lymph glands, we may conclude that they are the chief sources of these white cells. The chyle of the lacteal vessel of the mesentery contains particles of fat which greatly exceed in size those found in the thoracic duct, so we may infer that the fat emulsion undergoes a further subdivision or modification on its way through the glands.

Lymph which has been collected from the lymph channels of the extremities is an almost clear, colorless fluid, rich in the waste products of tissue change, but containing less albumin than that coming from the main trunk, and no fat. After long fasting the lymph from the thoracic duct has the same characters.

Lymph contains a considerable quantity of carbonic acid gas, about 50 vol. per cent., some of which is readily removed by the air pump, and is therefore said to be absorbed by the fluid, while some can only be removed by the addition of acids, and therefore is considered to be in chemical combination. Only mere traces of oxygen have been found in the lymph.

The quantity of chyle and lymph poured into the blood varies so much that any estimation of the amount entering in a given time is unreliable. •

The following circumstances upon which the variations may depend are instructive: -

1. The ingestion of liquid and solid food causes a great increase in the amount of chyle. This is obvious from the change in the state of the lacteal vessels, which, from 18 being transparent and almost empty, become widely distended and white.

2. The activity of any organ causes an increase of lymph to flow from it.

3. Impediment to the return of the venous blood from any part increases the irrigation, and hence the lymph.

4. Increase of the amount or the pressure of the blood flowing through any part augments the lymph flow.

5. The administration of curare increases the amount of lymph.

The history of the structural elements or lymph corpuscles which exist in such numbers in the large lymph channels, requires some further discussion, as these cells are composed of active protoplasm destined for some important function, and must be produced by some vital process.

The origin of the lymph corpuscle is not restricted to any one part of the body or to any special organ. It has been already said that the lymphatic glands are the most important source of these cells, because the follicular tissue is filled with them, and the lymph contains a much larger number after it has passed through some lymph glands. In the lymphoid tissue of the spleen and the intestinal mucous membrane they are very numerous, and; no doubt, many have their origin in the follicular tissue of that organ and intestine. They are said also to be formed in the red marrow of the bones. Although their number is relatively small, lymphatic cells occur even in those lymph channels that are unconnected with a lymphatic gland, and these cells, no doubt, come from the blood, which contains many cell elements, identical with the lymph cells found in the lymphatic duct. These cells, when they arrive at the minute blood vessels, sometimes leave the vessels and creep by amoeboid movements into the interstices of the tissue with the irrigation stream. They may permanently abide in the tissue, or be washed back into the larger lymph channels with the surplus stream of lymph. When the abnormal increase of activity in a tissue known as inflammation occurs, this escape of the white cells from the blood takes place with great rapidity, and the stages in the process can be watched under the microscope.

Still another source of the lymph cells may be from proliferation of the cells which lie in the tissues. The fixed tissue cells are said to be capable of producing elements identical with lymph cells, which by division possibly multiply and produce their like, and may be carried along by the lymph stream as lymph cells.

The enormous number of cells which accumulate as pus when an abscess forms are structurally identical with lymph cells, and probably arise from these combined sources, viz., escape from the blood vessels and proliferation of the tissue cells.

The lymph cells, therefore, whether they have their origin in a lymph gland, spleen, or connective tissue, perform a kind of circuit, going with the lymph into the blood, and are distributed with the latter to the tissues, whence they may be once more carried into the lymph stream.