The liquid portion of the exudation represents something more than the transuded blood-serum, and a certain practical importance results from the distinction drawn between an exudation and a transudation. Such a distinction is especially called for when the inflammatory or non-inflammatory origin of considerable quantities of fluid in the larger cavities of the body is concerned. From a recent contribution to our knowledge of this subject by Reuss[13] the following information is derived: The percentage of albumen is always greater in exudations than in transudations, and is more constant in the former than in the latter. It increases with the severity of the inflammation, being highest in the ichorous forms, less in the purulent, and least in the serous exudations. When an {43} inflammatory exudation is found to contain less albumen than usual, the existence of a transudation with secondary inflammation is suggested, or the exudation may have taken place in a hydræmic individual. A sufficient number of exceptions are met with, however, to interfere with the absolute nature of this test. [Footnote 13: _Deutsches Archiv für Klinische Medicin_, 1879, xxiv. 583.] The coagulation of an inflammatory exudation apparently depends upon the contained white blood-corpuscles; the more numerous (within certain limits) these are in a serous exudation, the more abundant is the formation of fibrin. The cellular element likewise is that which in abundant liquid exudations characterizes them as purulent. Although it is generally agreed that most of the corpuscles of pus are emigrated white blood-corpuscles, it is not necessary to admit that all are of this nature. The cells present in an inflamed part include those pre-existing, as well as those which escape from the vessels. The former are the wandering cells of the connective tissues, as well as the fixed variety, the epithelial cells of the surface of a mucous membrane in addition to the subjacent connective-tissue cells. Amoeboid cells outside the blood-vessels have been seen to divide, and it is possible that such duplication may serve as the method of formation of a certain number of pus-corpuscles. The statements concerning the proliferation of the fixed connective-tissue cells and of epithelium are derived from appearances, and are interpretations of these appearances, not observations of a process.