If the lymph is collected from an animal fed some time pre viously with a fatty meal it has a milky appearance due to the presence of innumerable fine drops of fat about .5 to .75A in diameter. This fat is absorbed by the lymphatics of the small intestine, which because of their white appearance during diges tion are called lacteals.
If, however, lymph is collected separately from various organs and limbs it is found to differ very considerably in its protein con tent. Thus lymph collected from the liver is very little inferior in protein content to that of blood, while that of leg lymph is considerably less. These two examples represent the extremes of difference, samples from other organs falling between these values.
In the dog as much as o•6 litres, in the cow about 2 litres of lymph per 10 kilos of body weight is eliminated by the thoracic duct in 24 hours. Direct measure ments have not been made on man but probably 4 to 5 litres —equal to the whole blood volume—passes through the duct in a day.
The formed elements of lymph consist almost entirely of lymphocytes which are added to it in its pas sage through the lymphatic glands, and from the lymphatic tissue such as is present In the spleen, intestine and tonsils. The actual number present at any time is very variable, in a cat usually 10, 000-20,000 per c.mm. in the lymph from the intestine. It has been found that mechanical pressure on lymphatic glands is capable of increasing this number very considerably, so that the lympho cytosis in the blood after exercise can be explained on this basis alone. Nevertheless there is some evidence that contraction of the smooth muscle contained in the gland capsule can also force lymphocytes into the lymph passing through the gland. Under normal conditions red cells are absent or, in the dog, present to the extent of i,000 to 2,000 per c.mm. A few polymorphonuclear leucocytes and macrophages are also found.
A tissue consists of an agglomeration of cells in intimate contact with the tissue fluid surrounding them. The blood conveys to it nutrient and other substances both in simple solution and in chemical combination with certain of its elements. The transfer of these substances
takes place through the blood capillary wall. The forces sub serving this transference would appear to be of a physical charac ter though the capillary endothelium, being composed of living material, is capable of altering its degree of permeability. This endothelial wall appears at all times to be freely permeable to dissolved salts, but its permeability to proteins is a variable fac tor. The physiological conditions under which this permeability alters are not known, but under pathological conditions, such as are produced by mechanical injury, the proteins may pass freely through the injured endothelium.
forces producing a transference of fluid and dissolved substances from the interior of the capillary to the tissue spaces are blood pressure, diffusion and osmosis. The hydrostatic pressure existing in a blood capillary over that in the extravascular tissue spaces causes a filtration through the endothelial wall of fluid having the same composition in salts as blood but varying in protein content. At the same time diffusible substances at a greater concentration in the blood tend to pass to a region of less concentration in the tissues. The third force, that of osmosis, also comes into play owing to the fact that water is capable of passing through the capillary membrane more quickly than salts,—thus if a hypertonic solution of sodium chloride is injected intravenously there is a passage of water from the tissue to the blood which is greatly increased in volume, with a concomitant desiccation of the tissues. At the same time salt passes more slowly into the tissues till a new equilibrium is reached. In the reverse direction an increased molecular con centration in the tissues is capable of drawing fluid from the blood. These factors are sufficient to account for a passage of fluid and dissolved substances from the blood to the tissues. The products of tissue metabolism, however, have to be removed to gether with excess water and salts, the former of which has been the vehicle for the conveyance of nutriment.