Another classification has been proposed, of which the foundation is the degree of re semblance of the secreted products to the normal constituents of the blood ; those being associated into one group, whose charac teristic ingredients are altogether unlike those of the blood ; and a second group being formed of those, whose elements seem nearly allied to those of the blood. This classification is practically almost the same with the pre ceding ; for, as we shall hereafter see, all the cases in which the secreted products are very unlike the constituents of the blood, are those in which they are most directly and speedily removed from the body ; whilst those in which they serve some ulterior purpose, are for the most part also those, whose elements differ least from the components of the blood.
The first group of these processes cor responds with that which has been elsewhere treated of under the head of EXCRETION ; and the resultant products have been termed excrenientitious secretions, or more briefly ex cretions, in contradistinction to the recrenzen titious secretions, which are the products destined for ulterior uses.
There is another group of processes, which corresponds so completely vvith the secreting operations in its general nature, that it is diffi cult to avoid placing it under one category with them; the more especially, as the instruments by which it is effected correspond with the organs of secretion in the most essential fea tures of their structure. We refer to that elaborating agency, which is now generally believed to be exerted upon certain materials of the blood by the spleen, thymus, and thy roid glands, and suprarenal capsules (which are sometimes collectively termed vascular glands), and also by the glands of the ab sorbent system. The " vascular glands," as will presently appear, exactly correspond with ordinary glands in all that part of their struc ture by which they withdraw or eliminate certain matters from the blood ; and they differ only in being unprovided with excretory ducts for the discharge of the product of their operation. These products, instead of being carried out of the body, are destined to be restored to the circulating current, apparently in a state of more complete adaptiveness to the wants of the nutritive function ; in other words, these vascular glands are concerned in the assimilation of the materials that are destined to be converted into organised tissues, instead of being the instruments of the re moval of the matters which result from the disintegration or decay of those tissues. And
in regard to the entire absorbent system, with its glandulm, reasons will be presently ad vanced for regarding it all as one great secre tory apparatus, whose relations are essentially antagonistic to those of the excreting appa ratus ; the materials of its operation being de rived from the external world, and its products being poured into the blood ; and its purpose being to supply fresh pabulum to the circu lating fluid, whose effete matters are being drawn off by the eliminating agency of other glands, whose products are carried back to the external world.
The line of demarcation between the func tions of nutrition and secretion can scarcely be drawn with definiteness ; so close is the affinity between the two sets of operations, both in their nature and in their purpose. For, as will presently appear, every act of true secretion is really a part of the nutritive pro cess, the selection of the materials on vt hich the secreting organ acts being effected by the development of certain groups of cells, which, during their short period of existence, form a part of the solid constituents of the body ; so that, as was first pointed out by Professor Goodsir, the functions of nutrition and secretion are essentially the same in their nature. In regard to the objects of the two functions, moreover, there is not that differ ence which might at first sight appear ; for although the nisus of the nutritive functions is directed towards the increase and mainte nance of the solid fabric, and that of the secreting operations to the removal of certain fluids from the circulating current, the reten tion of which would be injurious, yet here again there is much common ground. For, as was first pointed out by Treviranus, " each single part of the body, in respect of its nu trition, stands to the whole body in the rela tion of an excreted substance ;" in other words, every part of the body, by taking from the blood the peculiar substances which it needs for its own nutrition, does thereby act as an excretory organ, inasmuch as it removes from the blood that which, if retained in it, would be injurious to the nutrition of the rest of the body. Thus the phosphates which are deposited in our bones are as effectually ex creted from the blood, and prevented from acting injuriously on the other tissuus, as are those which are discharged in the urine.