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

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.

MOVEMENT OF THE LYMPH.

In some of the lower animals small muscular sacs occur in the course of the main lymph channels, which pump the lymph into the great veins by contracting rhythmically, much in the same way as the heart.

In man and the higher animals no such lymph hearts have been found; the onward movement of the fluid depends chiefly on the pressure under which the irrigation stream leaves the blood vessels. The fluid in the blood vessels, as we shall presently see, is under considerable pressure, which causes the plasma to leave the capillaries. Hence, if a lymphatic trunk be tied, its tributaries are filled with lymph until a considerable pressure (8-10 mm., soda solution) is developed in their radicles.

While the pressure exerted on the small tributaries of the lymph channel may become considerable, that in the thoracic duct is invariably very low, for the following reasons: The blood in the large veins into which the duct opens is under less pressure than in any other part of the vascular system, owing to the thoracic suction, or negative pressure in the thorax, caused by

the elastic traction of the lungs. In fact, the pressure in the large veins, e. g., brachial, etc., varies from o to 4 mm. Hg., and that in the vena cava is always negative, except in sudden or forced expiration, and varies, according to the period of the respiratory rhythm, from 5 mm., in inspiration, to

2 mm. in

Diagram showing the Course of the Main Trunks of the Absorbent System. The lymphatics of lower extremities (D) meet the lacteals of intestines (LAC) at the receptaculum chyli (R. C.), where the thoracic duct begins. The superficial vessels are shown in the diagram on the right arm and leg (S), and the deeper ones on the arm to the left (D). The glands are here and there shown in groups. The small right duct opens into the veins on the right side. The thoracic duct opens into the union of the great veins of the left side of the neck (T).

The fact that the lymph at the origin of the small channels is at a pressure of 8 to 10 mm. of water, while at the entrance to the vein it is mil, would be sufficient to explain the movement, even if there were no other force aiding it.

It must be remembered that every lymph vessel is furnished with closely set valves, which prevent the fluid it contains from being forced backward, so that any accidental local pressure exercised on the exterior of a lymph channel helps the fluid onward to the veins. Along their entire extent these vessels are subject to certain forces which must materially aid the flow of the lymph stream. The first of these is the pressure exerted on the small vessels by the movement of the muscles in the neighborhood. The second is the unequal distribution of atmospheric pressure, which has full force on the peripheral, channels, but is kept off the thoracic duct and its termination, as already mentioned, by the rigidity of the thoracic wall, which, together with the tendency of the elastic lungs to shrink, causes a permanent negative pressure in the thoracic cavity through which the duct passes. And, lastly, the thin-walled lymphatics are everywhere surrounded with very elastic textures enclosed in an elastic skin which exert an amount of pressure sufficient to empty and press together the walls of the vessels after death, and therefore during life must have considerable influence upon the fluid they contain.

The movements of the chyle depend on the same forces, with the addition of the power used in the contraction of the villi, which pump the chyle from the lacteal radicles into the network of valved vessels in the submucous tissue.

The commencements of the thoracic duct and the lacteals are placed in the abdominal cavity, and therefore are constantly. under the influence of the positive pressure exerted by the abdominal wall on the contained viscera. The rest of the duct is in the thorax, where the pressure is habitually negative. Certain variations coincident with inspiration and expiration take place in both these cavities, and must aid the onward flow of fluid in a vessel containing valves so closely set.