In order to understand these conditions it is necessary to refer to certain points in the normal relations of the lymphatic circulation. The connective tissue throughout the body is as it were permeated with spaces of various shapes in which the connective tissue corpuscles lie. These lacunae or serous spaces are lined with endothelium like the blood-vessels and lymphatics themselves. The serous spaces are provided (as shown in Fig. 35) with numerous anastomosing processes, and they communicate on the one hand with the interior of the bloodvessels, and on the other with the lymphatics. Arnold and others (see Fig. 36) have succeeded in injecting the serous spaces from the bloodvessels, especially when, as in passive hypersemia, the channels of communication between blood-vessels and spaces have been widened. A circulation is continually proceeding from the capillary blood-vessels into these spaces, and so into the capillary lymphatics. The serous cavities of the body are to be regarded as large serous spaces. They also are lined with endothelium, and are continuous with the lymphatic capillaries. To a considerable extent also the lung alveoli are similar to the serous spaces. We know that fine dust inhaled into the alveoli readily passes into the lymphatics, and in cedema of the lung the fluid is in the alveoli.
(Edema is overfilling of the serous spaces with fluid, dropsy is overfilling of the serous cavities, but the latter term is often used in a general sense to include both. When cedema affects the body generally it is often called general dropsy, or Anasarca or Hyposarca. Opposed to this are local cedema and local dropsy.
Anything which causes an excess of fluid in these spaces or cavities will produce oedema or dropsy, and the first condition which suggests itself is obstruction of the lymphatics. The force which carries on the lymphatic circulation is largely the blood-pressure propagated from the capillaries, and it may be expected that obstruction to a lymphatic stem would produce accumulation of fluid in the spaces. But it is to be remembered that the serous spaces are in as close relation to the blood-capillaries as they are to the lymph-capillaries, and any obstruction to the lymphatics will probably have the effect of causing the transuded fluid to return to the blood-capillaries. Majendie demonstrated that the power of absorption possessed by the veins is very great, and experiment has shown that the whole lymphatics of a limb may be ligatured, or what is equal to that, the entire lymphatic glands excised without producing oedema. The blood-vessels take up the functions till new lymphatic channels are formed.
Fig. 35. - Serous spaces (c, c) and lymphatic capillaries (d, d) on pleural surface of diaphragm in preparation treated with nitrate of silver. The connection of the branched spaces with the commencing lymphatic vessels is shown. (Recklinghausen).
When the Thoracic duct is obstructed there is not necessarily any oedema or dropsy, although lesions which obstruct the duct generally interfere also with the veins and predispose to oedema. On the other hand, when the duct is obstructed there is distension distal to the seat of obstruction, and this may lead to rupture of the chyle vessels in the abdomen or of the duct itself. In this way occur Chylous ascites and Chylous pleural exudation. Recklinghausen has also seen a case of chylous ascites arising from obstruction of the chyle vessels of the mesentery with cancerous growth. Manson, again, accounts for the occurrence of chylous urine and lymph scrotum by obstruction of the lymphatic glands with the embryoes of the filaria sanguinis (see under Parasites). We may say, however, that with these exceptions, obstruction of lymphatics does not of itself lead to oedema or dropsy, although it is clear that it may aggravate an oedema whose cause is to be found in some other condition.
Fig. 36. - Capillary blood-vessels, serous spaces and capillary lymphatics of the frog's swimming web, filled with (blue) injection material thrown into the bloodvessels. (The black branched bodies are the normal pigment cells.) a, a, Capillary blood-vessels filled with injection material, b, b, Lymphatic capillaries also injected but less full than the blood capillaries, c, c, Serous spaces injected from the bloodvessels. The injection was made after passive hyperemia had been produced by ligaturing the vein. (Arnold.) x250.
Passive hyperaemia is the most frequent cause of oedema and dropsy. Here there is increased pressure in the capillaries and veins, and a greatly increased transudation. From what has gone before, it will appear probable that the increase of pressure is not the only element in the case, but that the capillary wall is so altered as to be abnormally permeable. Indeed, alteration of the capillary wall is evidently the more important, as shown by the fact, already referred to, that oedema is most likely to occur where from weakness of the heart the circulation specially sluggish. It is so, for example, in hypostatic hyperaemia.
The observations of Lazarus-Barlow indicate that increase of venous or of lymphatic pressure alone is insufficient to produce oedema, and that in the oedema of passive hyperemia insufficiency in the supply of blood or in the quality of the blood is the essential element. This view appears more probable when we find that Active hyperemia rarely produces oedema unless complicated with inflammation, in which case the agent which causes the inflammation presumably acts on the capillary wall. (See under Inflammation.) In passive hypersemia the fluid exuded is a watery serum, and as there is diapedesis it contains red corpuscles in considerable numbers. The red corpuscles are taken up by the leucocytes as already described, and carried to the lymphatic glands where they disintegrate.