He divided the lymphagogues into two groups. Those of the first are proteins or protein degradation products, for example, extract of mussels, crabs, strawberries, and peptone. Those of the second group are of a salt character, for example sodium chloride, which are possessed of considerable osmotic qualities. The injection of lymphagogues of the first order produces an in creased flow of lymph, mainly from the liver. It has been pointed out that substances of this nature when pricked into the skin cause wheals. Recent work has shown conclusively that the fluid content of these wheals is derived from the blood through a cap illary wall which has become more permeable.
It has therefore been argued that this type of lymphagogue in jures the blood capillary endothelium and leads to an increased filtration of fluid and thence of lymph. It should be noted that such substances as peptone stimulate the liver cells so that this may be an added f actor in the increased lymph production. Hei denhain, on the other hand, maintained that these substances specifically excited the lymph capillary endothelium to secrete lymph.
The action of the second type of lymphagogue has been cor related with the great osmotic activities of these substances. In jected salts at first attract considerable quantities of water into the blood stream. This increase of the circulating blood volume increases the intracapillary pressure which tends to force fluid back into the tissue spaces. The flow of lymph depends on the relative activities of these two factors. With this class of lymph agogue the flow is also in conditions of rest, mainly from the liver and intestines.
It has been shown that after injections of large quantities of saline solutions the muscles increase their water content very considerably, but there is no flow of lymph from the limb under such conditions. This has been held to refute the filtration-osmo sis theory of lymph production and to make essential the con ception of active secretion by the lymphatic endothelium. A point, however, which has not been considered is that skeletal muscle is not supplied with lined lymphatic vessels. These only exist in the perimysium—the tissue surrounding the muscle bun dles. Thus it is clear that muscle can take up large quantities of fluid which are not in actual contact with absorbing lymphatics, whereas organs such as the liver and intestines possess an ex tremely rich lymphatic capillary system.
It has also been urged that as injected salt accumulates to a greater extent in lungs and skin than in muscle, therefore a purely osmotic conception of its transfer is inadmissible. While this is
true, our ignorance of the phenomena and laws governing salt distribution in the body cells does not make it necessary to deny potency to forces demonstrably present. Attempts have been made to show" that injections of small quantities of salt solution have a stimulant action on tissue cells, leading to increase of the water exchange and general metabolism of an organ and so to increase of the lymph outflow.
That at least some portion of the lymphatic tree in every tissue has a like grade of permeability is made probable by the fact that bacteria and similar sized particles are readily taken up to be arrested in the nearest lymphatic gland. Whether the whole capillary system is equally permeable has not yet been ascer tained. If it is, the endothelial wall of the lymphatic has little influence, apart from any "secretory" power, in the matter of selective filtration through its walls. An interesting observation dealing with the permeability of lymphatic vessels to diffusible substances has been made by T. Lewis. If histamine or adrenalin is inserted into the skin by puncture, some of the substance is taken up by the lymphatics nearby. They both however again pass out through the lymphatic wall as is shown by the fact that the course of the latter is traced out in the one case by a reddish line of dilated capillaries and venules, and in the other by a white line due to a contraction of these vessels.