A standardized solution of a vital dye which escapes with some difficulty from the lymphatics of the ear of the mouse has been utilized in tests of the permeability of the lymphatic wall under various conditions. It has been found that this permeability is subject to great change. The slight pressure that suffices to prevent lymph flow from the ear,—an organ in which such flow goes on normally,—soon results in increased permeability of the obstructed lymphatics without as yet any perceptible dilatation of these vessels. Mechanical stimulation as for example a stroke with a blunt wire, or scratching so light as not to break the epidermis, results in a practically immediate, great increase in lymphatic permeability, which is sharply localized to the region pressed upon. This increase in permeability, though so great that even hemoglobin is let pass by the lymphatics, endures but a few hours. Warming the ear to 43°C. or exposure to mild sunlight increases permeability considerably. Slight chemical irritation increases it greatly, though not so much that particulate matter is let pass. The edema developing as result of lymphatic obstruction or mechanical, thermal, or chemical stimulation is preceded by and associated with a large increase in lymphatic permeability.
The facts are discussed in relation to their bearing upon fluid accumulation within the tissue. It is plain that influences within the realm of the normal suffice to increase lymphatic permeability and that those which lead to edema cause a very great increase in it. In proportion as this increase occurs the lymphatics cease to be channels demarcated by a semipermeable membrane. It seems certain that the changes must be in some part responsible for the local accumulation of fluid. There exist possibilities, on the other hand, of a correlation between the functionings of the blood and lymph vessels under certain pathological conditions, as during the resorption of edema.