OF SECRETION.
TnE literal meaning of the word secretion, is separation ; and it was used nearly in this sense by the older physio logists, who considered this function to consist merely in the separation from the blood of certain substances that were previously contained in it. By the moderns the term employed in a more technical manner, to designate the faculty by which certain organs produce from the blood substances different from the blood itself, or from any of its constitutents. In the following chapter we shall first give an account of the organs of secretion ; in the second place, we shall describe some of the more important of the secreted substances; and, lastly, we shall offer some re marks upon the hypotheses that have been proposed to explain the nature of the operation.
The organs of secretion arc so infinitely varied in their form and structure that it is impossible to give any descrip tion of them which shall not be subject to numerous excep tions; but, taking theni in their most perfect, and, at the same time, in their most general form, they consist of a rounded body, which from its shape has obtained the name of gland. A gland may be said to consist essentially of a nuniher of arteries, which ramify in various directions through a mass of cellular texture ; from these proceeds another set of vessels, which contain the secreted sub stance ; and these vessels commonly unite in one or more trunks which are named excretory ducts. The gland usually consits of distinct masses or lobes, which may be successively divided into smaller and smaller lobes, until we arrive at what have been termed the acini, the smallest portions into which a gland can be mechanically separa ted. The glands, in common with other parts of the body, have veins, nerves, and absorbents, but these do not ap pear to be immediately concerned in their appropriate functions.
There is still much obscurity concerning the intimate structure of glands, especially respecting the question whether there be any intermediate part between the se creting artery and the excretory duct. Malpighi suppos ed that the secreting artery terminated in a small cavity or follicle, which was the immediate organ of secretion. Ruysch. in prosecuting his delicate injections, supposed that he had disproved the doctrine of Malpighi, and that he had established the point, that there is no intervening follicle between the artery and the duct ; and as he profess ed to derive his opinion from the simple exposition of facts, without any view to theory, his account has been in general preferred to that of Malpighi. Upon the whole, however,
it may be questioned, whether in this, as in so many other controverted questions, either party be absolutely cor rect.
The structure which we have now been describing, be longs to the more perfect of the glands ; but the complete apparatus is comparatively seldom met with, even in those instances where what appears the most elaborate opera tions arc performed. In some cases we can detect nothing more than a essel leading to a pouch, in which the secre tion is lodged, and which is sometimes provided with an excretory duct, and is sometimes without one. Many sect etions are poured out on the surface of membranes, where no specific structure can be detected, except a small vessel which terminates externally ; and there are a num ber of cases, in which a substance is produced and separa ted from the blood, where we are not able to discover any appropriate apparatus for the purpose. We may observe that the nature of the substance produced, so far as respects its resemblance to any of the constituents of the blood, bears no relation to the simplicity of the apparatus by which it is for med. The saliva, for example, is secreted by a large gland, while no organ has been detected for the secretion of the adipose matter, yet the latter is unlike any part of the blood than the former.
Glands have been arranged by anatomists in different ways, sometimes according to their structure, and at other tines according to the nature of the substance which they produce ; but no arrangement has yet been formed to which there are not many objections. The older writers divided them into conglobate and conglomerate ; the first consisting, as they supposed, of one lobe only, the others of a number of lobes connected by cellular texture. It is not, however, always easy to draw the line of distinction between the two kinds ; and it is obvious that many of the most important secretions are produced by organs to which neither of these terms will apply. But we shall be better able to understand the structure of the glands when we have reviewed the various substances that are secreted by them, which was the second point that we proposed to consider.