A technic is described for the demonstration of lymphatic capillaries in living skin and for their study. By means of vital dyes injected intradermally these vessels can be rendered plainly visible. They form an extraordinarily abundant anastomotic web. The least scratch, one which does not penetrate through the epidermis, gives rise to such conditions that lymphatic absorption readily takes place from the abraded surface; and so close-meshed is the lymphatic web that an intradermal injection with even the finest hypodermic needle tears some of the constituent vessels open with result that they undergo direct injection. In many individuals much of the fluid introduced at an ordinary intradermal injection, like that made in the clinic, spreads through the superficial lymphatic network, whereas in others it tends to enter the deeper lymphatics at once, the difference being due to merely physical factors determined by skin texture. Normal flow along the skin lymphatics is rapid even when the body is at rest, dye introduced into the skin of the resting forearm reaching the axilla within a few minutes. The observations make plain the fact that every intradermal injection is an intralymphatic one, often preponderantly such, while furthermore every local injection into the skin becomes within a few minutes a general one, so rapidly is the introduced material transported to the blood.
The normal permeability of the skin lymphatics of man is approximately the same as that of the mouse. Tests indicate that in both instances the lymphatic wall behaves like a semipermeable membrane. The permeability of the human lymphatic wall like that of the mouse is subject to rapid and great changes. A stroke on the skin with a blunt instrument to produce a wheal, causes the lymphatic capillaries to become so permeable temporarily that dyes pass through their walls as if practically no barrier existed, instead of being held back for a greater or less period. Slight inflammation due to heat, ultraviolet light or bacterial products has a similar effect. So, too, has histamine. When fluid pours rapidly into the tissue from the blood, as when a wheal is formed, the lymphatics are compressed and their efficiency as drainage channels is interfered with.
These facts are briefly discussed in their bearing upon skin phenomena in general. The lymphatics cannot be disregarded in considering such phenomena, in which it is plain that they have a large share.