The pus-corpuscle has uniformly the same character in all descriptions of pus. The dis. tinction of the varieties above enumerated, therefore, can only be microscopically effected (if it can be effected at all) by means of su peradded elements ; and most valuable these are as diagnostic of its seat and production in many instances.
The varieties of pus comprehended in the class (B), differ from those in the class (A), in being inoculable, — a character dependent not upon any peculiarity of their cell, but upon the associated intangible " virus." Some of the varieties of the class A possess, how ever, what may be called namely, those in which certain parasites are present. The pus of scabies is thus to be propagated by means of its entozoon ; that of porrigo by its entophyte ; but it is clear that the associated pus has in reality nothing to do with the transmissibility of the diseases.* There are three semi-fluid matters, which it is important to distinguish front pus, namely, mucus, s(,ftened fibrin, and ftuid holding epithe lium in suspension. The distinctive characters most to be relied on are as follow: (a) Mucus. (1.) Pus mixes with water, being at first equally diffused through it, so as to give it a yellowish tinge ; subsequently, the corpuscles fall to the bottom, and leave the supernatant fluid clear and colourless. Mucus does not mix with water, but eventually renders it slimy. (2.) Pus forms an emulsion with acetic acid, from which, after a time, the nuclei of the corpuscles are thrown down as a yellow sediment, while the involucra are dissolved. Mucus iS coagulated by acetic acid, and forms a membranous flocculent mass without mixing witlr the acid; at the same time it becomes less slimy and more con sistent. (3.) Pus forms a ropy mass with the caustic alkalies, or with their carbonates. (B. Babington.) Mucus, on the contrary, is rendered thinner, and partially dissolved by them. (4.) Pus contains fat removeable by ether, sometimes in such quantities as to render it inflammable; mucus contains none. (5.) Air bubbles in pus collapse the moment they are formed ; in mucus they re main for a time—for days even— unaltered. (6.) Equal parts of concentrated sulphuric acid and pus form a dull brown-red solution, becoming paler and turbid by the addition of water ; mucus, on the contrary, forms a pale brown fluid with this acid, which remains clear and becomes colourless on the addition of water. (Brett and Bird.) (7.) According to Preuss, pus (as also tubercle) is distin guishable from mucus by containing iron (which may be shown by inceration and di grating the ash in gum, hydrochloric acid, di luted with five parts of distilled water, and then adding a few drops of ferro.cyanide of
potassium) : but in point of fact the presence of iron is due to accidental admixture with blood. (8.) Pus pressed between two plates of glass and held before a candle, presents an iridescent appearance ; no such effect is ob served with mucus. (Young ) The state of knowledge concerning the two alleged prin ciples, mucin and pyin, is too unsettled to allow of just inferences being drawn fi.om the presence or absence of either.
Various attempts have been made to dis tinguish pus and mucus by means of the proper corpuscle of each. The difficulty experienced in the detection of distinctive characters gra dually led to the suspicion that the corpuscle of both fluids might be one and the same thing; and the inquiries of several competent persons appear at length to have distinctly established the fact, that healthy mucus contains no special corpuscle, but that, under the very slighest irritation of a mucous surface, pus, with its special cell, is thrown out, which cell had been mistaken for one peculiar to the natural secre tion of mucous membranes. The presence of a bougie in the urethra for a very short time suffices to cause the production of nzuce/tus.* The abundance of epithelium-scales in mucus is sometimes a useful aid in the diagnosis : the nuclei of these scales set free may, doubt less, also have been sometimes mistaken for special corpuscles.
(b.) Softened The semi-liquid matter frequently found in the centre of co agula in the veins and heart, was long con founded (from its colour, consistence, and easy miscibility with water,) with pus ; noto riously so by MM. Gendrin, Andral, Cruveil hier, and Magendie. It had been more or less confidently affirmed, however, by MK Dupuytren, Burrows, Davy, and others, that this matter really consisted of softened fibrin, and not pus, when Mr. Gullivert gave support to this notion by pointing out the following peculiarities, distinguishing the substance in question from pus : 1. It is not rendered ropy" by caustic volatile alkali. 2. It presents DO iridescence when pressed between plates of glass before a candle. 3. Under the micro, scope it is mainly composed of a finely granular mass, and often contains large, irregular, flabby particles, with globules of various sizes. But these globules bear but a very small proportion in nuniber to those in pus ; and, on the addi tion of acetic acid, they soon disappear, except a few which seem more compact, and require a longer time for solution : they are probably altered blood-corpuscles. 4. Softened fibrin more readily becomes putrid than pus. Fibrin removed from the body and subjected to a blood-heat, begins to change into matter, such as that now described, in forty hours.