Among the general secreting and excreting surfaces, we must not omit the skin, as this integument performs an office, viz, that of transpiration or perspiration, which has always been considered or great importance. The skin is well furnished with exhaling vessels, which pour out through its numerous pores the matter of per spiration.
The matter transpired during insensible perspiration, seems to differ little from that fluid which we call sweat. Both consist chiefly of water, holding in solution carbo nic acid, and probably sonic other acid. and having mix ed with it a peculiar oily matter. Sweat has evidently a salt taste, and perhaps a little inuriate of soda. Sec CHEMISTRY and PHYSIOLOGY.
-Of Secreting Glands.
The name Gland has been given to a great variety of organs that (liar from each other in form, structure, situation, and functions. We have already seen it ap plied to certain protuberances or the encephalon, as the pineal and pituitary glands, to those roundish bodies which belong to the absorbent system, which are called lymphatic, and sometimes congkbate glands; and to se Vera! bodies the use of which has not been ascertained, as the thyroid, the thymus gland, &c. Bichat is for re stricting the term to those bodies which s.parate parti cuhir fluids from the blood by means of one or duets or canals. Under this head he arranges the sa livary glands, the lachrymal glands, the glands of bomius, the tonsils, the ceruniinous glands ol the ear, the breasts in women, the liver, the pancreas, the kid neys, the prostate gland, the testes, and the innumera ble mucous glands that accompany the mucous mem branes. All these are properly secreting glands, and they have been called conglomerate, to distinguish them from the lymphatic glands. We have ventured, in the preceding table, to introduce the spleen among the glands, as it is now proved that it is analogous to the kidneys in its office.
Some of the secreting glands are superficial, as the br,ast and the salivary glands, but the gi ewer number are deeply scat d within the principal cavities of the body. St Ve ra. of them are single, as the liver, pancreas, &c. but in general they appear in pairs, those of each pair resembling each other in form, structure, and situ ation, but not always in size.
Glands (Idler in their intimate structure from most other organs of the body, in having nothing of the fi brous texture. Their component parts form an aggre gated mass, connected by cellular membrane, and adhe ring but loosely to each other. Sometimes they arc composed of insulated lobes, separated from each other to a considerable distance, by the intermediate cellular membrane; sometimes the glandular grains are equally and regularly arranged, and placed very near each other, and in a few cases their texture is uniformly pulpy, without any appearance of lobes or granulations.
Ruysch appears to have proved by minute injections, that the internal substance of the secreting glands is entirely vascular, consisting principally of a secreting artery exceedingly ramified and convoluted, and termi nating by its capillaries partly in a vein, and partly in an excreting duct. Other anatomists have supposed that cavities more or less extensive lie between the termi nations of the arterial branches and the excretory ducts; and that within these cavities the principal changes that form the secreted fluids, take place. Alalpighi con ceived that there existed in the glands, small bodies of a peculiar nature, that constituted the essential secret ing organs. These have been called crynta., but are supposed by Air Hewson to be only convoluted vessels. None of these points are, however, satisfactorily ascer tained ; we only know that the arteries which penetrate the glands communicate with the excretory ducts that pass out from them ; but whether this communication be direct or indirect, we cannot determine.
All the true secreting glands (following the system of Bichat) arc Furnished with membranous tubes, for the purpose of conveying the secreted fluid, either into particular cavities, or to the surface of the body. These tubes are the excretory ducts of anatomists. They arise, like the veins, by an infinite number of capillaries, which form greater and greater ramifications, till they unite in the general trunk. Where granulated bodies exist in the gland, each of them appears to give origin to one of those capillary ducts. The ramifications pass in straight lines from the inmost recesses ol the gland towards the general trunk of the duct. The excretory ducts are composed principally of two layers, a central mucous membrane, which may be considered as a pro deletion from that which lines the cavity into which the duct opens, or front the skin; and a peripheral mem brane Is hich differs both in thickness and appearance in dillerent All the glands are furnished with blood-vessels, but these vessels enter the gland, and go out Irvin it in a different manner, according as the glands are inyeloped in a general membrane, ur consist of separate and dis tinct lobes. In the former case, the arteries enter, and the veins conic out at one side ol the gland, where there is commonly a fissure or depression ; in the latter case, the arteries penetrate the glands on all sides, and the veins conic out in a similar manner. glands arc well supplied with nerves, but it does not appear that their function and secretion are influenced by the ner vous power. Sec Bichat, Gen. win. iv. p. 605. The glands have absorbents, and probably exhalants, but these are confined chiefly to their peripheral surface.