Before we attempt to give an account of so numerous a class of bodies as the secretions, it will be necessary to adopt some kind of classification of them. The earlier anatomists divided them into recrementitious and excre mentitious; the first comprehending those substances, which, after they are formed, serve some useful purpose in the system ; the second, such bodies as are secreted for the purpose of being rejected, as either useless or noxious. There is a foundation for this division, but it is neither suf ficiently comprehensive nor minute to be of much advan tage in our investigations respecting the nature of the substances. As the knowledge of physiology advanced, more scientific arrangements were constructed, founded upon the chemical or physical properties of the substances, but still they were all incorrect or defective, Haller class ed these secretions under four heads ; the aqueous, the mucous, the gelatinous, and the oily. Fourcroy, in confor mity with the improved state of chemistry when he wrote, formed them into eight divisions, the hydrogenated, the oxygenated, the carbonated, the azotated, the acid, the sa line, the phosphated, and the mixed. This arrangement, although it may be considered as more scientific than any which had preceded it, and in some measure corresponds with the improvements of modern chemistry, is yet, upon the whole, rather founded upon theory, than upon the ac tual nature of the substances concerned.
There is one question, which it will be necessary to in quire into in this place, whether there may not be certain bodies to which the term of secretions literally applies, according to its original acceptation, viz. substances which exist ready formed in the blood, and which are merely separated from it, without undergoing any change in their chemical composition. This we may conceive to be the case with the muscular fibre, for it would appear that it resembles the fibrin of the blood in every respect, except its mechanical structure, the muscle having its particles regularly arranged and organized, whereas the fibrin is dissolved or suspended in the blood without any regular or ganization, a circumstance which necessarily results from the latter being in a fluid state. The same remark ap plies to the earthy salts which compose the bones, for it may be conceived to be more probable that these are merely separated from the blood, than that they undergo any chemical change through the action of the vessels by which they are deposited.
There are likewise a number of fluids, respecting which it may be doubted, whether they are properly entitled to be considered as or whether they are only sepa rated from the blood by a kind of transudation. Of this kind is the fluid in the pericardium, and the different spe cies of dropsical effusions, which would appear to contain the same ingredients with the serum of the blood, except that they are diluted with a greater quantity of water.
Had we a perfect knowledge of the nature and compo sition of all the secretions, the most correct method of arranging them would be, according to their chemical composition or their chemical relation to the blood ; and although at present any attempt of this kind must be ne cessarily imperfect, yet, as this is the case with every other arrangement that has been proposed, we think it proper to employ it as the least exceptionable. On the subject of ar rangement there are a few preliminary remarks which we must attend to. We may observe that most of the secre tions consist of more than one ingredient, and that the properties of the compound occasionally depend so much upon both the substances that enter into their composition that it is difficult to determine which is to have the pre ference. We may remark, in the second place, that the same gland often furnishes a substance of very different properties in different states in the system, and this with out the occurrence of what can be strictly termed disease. Then it is obvious that the nature of the secretion is ma terially affected by disease, and from this cause new sub stances are occasionally formed that did not previously ex ist. In order to take a perfect view of the subject, we should mark the gradations front the healthy to the morbid condition, and observe all the intermediate states which the secretions assume ; this, however, would require a more advanced state of the science than we at present possess.
Making a due allowance for all these circumstances, and excluding those substances which appear to be merely se paratect from the blood, without undergoing any change, we may arrange the secretions into seven classes; the aqueous, the albuminous, the mucous, the gelatinous, the oleaginous, the resinous, and the saline.
The aqueous secretions are those that consist almost entirely of water, or of which the water composes so large a proportion as to give theni their specific charac ters. The most important of these is the matter of per spiration which is thrown off from the surface of the body. Attempts have been made to collect the cutaneous perspi ration, in order to examine its properties ; and from these it would appear to consist of water, containing animal and saline matter, the quantity of which is so small as to ren der it very difficult to ascertain their nature. Probably no other substance strictly belongs to the class of the aqueous secretions; for the exhalation from the lungs is to be regarded as nothing more than water evaporated from the mucus which lines the pulmonary vesicles. We are entirely ignorant of the nature of the apparatus by which the cutaneous perspiration is produced ; and indeed there is some reason to doubt whether it is not to be regarded rather as the effect of transudation than of secretion.