These observations suggest the following conclusions :—Sub stances may first be absorbed from the tissues by the lymphatics and then, during their passage down these channels, pass out again into the surrounding tissue fluid to be re-absorbed by the blood vessels. Consequently the composition of lymph collected from a lymphatic trunk does not necessarily represent that from a lymph capillary.
It has been shown by Hicks that the active principle of the thyroid gland is absorbed in part at least, by the lymphatics of that organ.
It is probable that the less diffusible chemical products of other organs of internal secretion are also absorbed in the same way, but only in the case of the thyroid gland is such evidence f orth coming.
To sum up concisely the views generally held as to the mech anism of lymph formation, it may be said that the physical forces of osmosis and filtration are largely responsible for the inter change of fluid and salts between blood, tissue and lymph. The metabolic activities of the tissue cells are also an important factor in determining both the quality and quantity of lymph outflow.
The permeability of the blood capillary wall partly controls the composition of the tissue fluid and lymph resulting therefrom : the influence of lymphatic permeability is not known. The balance of evidence is against endowing the lymphatic endothelium with specific secretory properties.
The propulsion of blood and lymph is effected by very different agencies. The blood vascular system forms a completely closed circle through which fluid is pumped by the heart continuously in one direction. The lymphatic system begins as blind ended capillaries which unite, finally to pour their contents into the venous system. Thus there is no circulation of lymph in the same sense as there is of blood, but there is movement of lymph from the periphery, represented by the lymph capillaries, to the centre represented by the venous system. The factors involved in this propulsion have to be ex amined. The forces concerned may be grouped into those of extrinsic and of intrinsic origin.
Of the greatest importance from a me chanical point of view is the fact that all the larger and most of the smaller trunks are liberally supplied with valves. These usually consist of two folds of endothelium, though occasionally more cusps are encountered. They are set so as to prevent flow
towards the periphery. They have long cusps which are particu larly efficient in preventing reflux and it is by virtue of their pres ence that muscular contraction or massage forms one of the chief propulsive agents. When a muscle contracts it squeezes the lymphatic vessels, thus tending to empty them in the direction permitted by the valves. That this mechanism is of very great importance can be seen if the lymph flow from a canula inserted into a lymphatic of a limb be examined. In a condition of rest no lymph flows from the canula, but if the limb is massaged or the muscles stimulated to contract actively, there is a considerable flow.
From organs such as the liver and intestine there is a continuous flow of lymph at all times. That from the liver is obviously independent of muscular contraction. It has been shown that the flow from these organs is in close relation ship with the blood pressure in their capillaries, the continuous outflow of lymph being due to a transmission of this pressure by way of the tissue fluid. In the case of the intestine the pressure in the main efferent lymphatic may be as much as 7 to lc) cm. of water. It is noteworthy that liver and intestinal lymph is by far the richest of any in protein, from which it is argued that the blood capillaries of these regions are very permeable, thus offering the best conditions for a transference of the hydrostatic pressure within them via tissue fluid to the lymphatics. In the limbs where the lymph has a low protein content, due to a relative imperme ability of the blood capillaries, the driving power of the blood pressure is minimal.
It follows that any increase of pressure in the tissue fluid causes more of the latter to be thrust through the lymph capillary wall, thus massage, apart from its action on the main valved trunks, forces fluid from the tissue spaces into the lymph capillaries. This can be especially well seen in the case of the lymphatics of the diaphragm, where it has been shown that increased intra-abdominal pressure from mas sage or active movements has a very considerable effect in forcing both fluid and particles into the lymph capillaries.