Dropsy arises when the transudation is accumulated. As dropsical accumulations are transudations from the blood, essentially blood-serum with a diminished percentage of albumen, and as such blood-serum is practically lymph from its presence in the lymph-vessels, dropsical effusions are to be regarded as stagnant lymph. Such stagnations may be present in the small lymph-spaces within the connective tissue, or in the larger lymph-sacs, as the peritoneal, pleural, pericardial, and scrotal cavities. In like manner, the stagnation may take place in the cavities of joints and in those of the brain and cord, although the latter represent functional rather than structural lymph-canals. The term oedema is applied to the accumulation in the connective-tissue lymph-spaces in general, while the term anasarca is confined to those cases where the subcutaneous lymph-spaces are concerned. The accumulation in the great lymph-cavities is known as ascites when peritoneal, hydrothorax when pleural, hydropericardium when pericardial, hydrocele when in the cavity of the tunica vaginalis, hydrocephalus if within the ventricles of the brain, and hydromyelocele when within the central canal of the spinal cord. The accumulation of dropsical effusions may be considered as possibly resulting from an obstruction to the channels through which the transudation should flow, or from insufficient force to overcome normal obstructions, or from an abnormally increased transudation. Lymph-channels are frequently obstructed, but no appreciable diffused retention of lymph results unless the thoracic duct is obstructed. This rare affection is followed by enormous distension of the thoracic and abdominal portions of the parts beyond the stenosis. Ascites and hydrothorax may follow, but not necessarily any considerable oedema of the peripheral parts of the body. As a result of the distension of the thoracic duct, rupture is not unlikely to take place, and the effused fluid contains chyle.[26] [Footnote 26: Quincke, _Deutsches Archiv für Klin. Med._, 1875, xvi. 121.] {70} That the obstruction is not followed by oedema is attributable to the innumerable anastomoses between the lymph-spaces, and also to the probability that a part of the transuded fluid returns to the blood-vessels when the obstruction is impassable. The forces necessary to promote the flow of lymph have already been mentioned, and their entire removal is inconsistent with life. A diminution of their activity is more likely to result in a diminished flow of lymph than its accumulation, although a slowing of the lymph-current may represent a favoring element in the accumulation of an increased transudation.