The application of this idea has been thus felicitously extended by Air. Paget*:—" The influence of this principle may be considered in a large class of outward growing tissues. The hair, in its constant growth, serves, over and above its local purposes, for the advantage of the whole body ; in that, as it grows, it re moves from the blood the bisulphide of pro teine, and other constituents of its substance, which are thus excreted from the body. Now this excretive office appears, in some in stances, to be the only one by which the hair serves the purpose of the individual ; as, for example, in the fo3tus. Thus, in the fcetus of the seal, and I believe of most other maminals, removed as they are from all those conditions against which hair protects, a perfect coat of hair is formed within the uterus, and very shortly after birth is shed, and replaced by another coat of wholly different colour, the growth of which had begun within the uterus. Surely, in these cases, it is only as an ex cretion, or chiefly as such, that this first growth of hair serves to the advantage of the individual. The lanugo of the human fcetus is an homologous production, and must, I think, similarly serve in its economy, by re moving from the blood, as so much excreted matter, the materials of which it is composed.
" Now if this be reasonable, we may carry this principle to the apprehension of the true import of the hair, which exists in a kind of rudimental state on the general surface of our bodies, and to that of many other permanently rudimental organs, such as the mammary glands of the male and others. For these rudimental organs certainly do not serve, in a lower degree, the same purposes as are served by the homologous parts which are completely developed in other species, or in the other sex. To say they are useless, is contrary to all we know of the absolute perfection and all-pervading purpose of creation ; to say they exist merely for the sake of conformity with a general type of structure, is surely unphilo sophical ; for the law of unity of organic types is, in larger instances, not observed, except when its observance contributes to the advan tage of the individual. No ; all these rudi mental organs must, as they grow, be excre tions serving a definite purpose in the economy by removing their appropriate materials from the blood, and leaving it fitter for the nutrition of other parts, or adjusting the balance which might else be disturbed by the formation of some other part. Thus they minister to the self-interest of the individual ; while, as if for the sake of wonder, beauty, and perfect order, they are conformed with the great law of unity of organic types, and concur with the universal plan observed in the construction of organic beings."
We cannot have a better example of the close affinity between the functions of nutri tion and secretion, in regard alike to their essential nature and to their purpose, than that which is afforded by the structure, growth, and offices of the adipose tissue. Fat, wherever it exists, whether in large isolated masses, or dispersed through areolar tissue, is made up of an aggregation of minute cells, whose peculiar province it is to draw into themselves the superfluous oleaginous matter of the blood, as a part of the history of their own development. Since they form constituent parts of the organism, and may possess as great a duration as that of any other of the elements of the soft tissues of the body, the growth of fat cells is commonly regarded as an act of nutrition. But it may also be considered as an act of secretion ; for it is the means of separating from the blood a product which is not destined to undergo any further organisation, and whose accumulation in the circulating fluid, beyond a very small and limited amount, would be positively noxious. This very same act of elimination of fatty matter, when performed by the cells of the liver, or of the sebaceous follicles of the skin, or (abnormally) by those of the kidney or of the intestinal glandulae, is recognised as forming part of the function of excretion, the difference being simply in the position and re lations of the secreting cells. For whilst those of' the glands are placed upon or near the free surfaces of follicles or ducts, and are destined from the first to a speedy exuviation, those of fat are woven up with areolar fibres and membranes, and form solid masses of tissue. A distinction might be drawn, on the ground that the contents of the fat cells are destined to be again taken into the circula tion ; whilst those of glandular cells, having been once eliminated from the blood, are never to return to it. But this would not hold good ; for the fat cells appear to have an in definite duration, the reception of their con tents into the circulating current seeming entirely to depend upon the demand for these in the blood* ; and there is now sufficient evidence that a considerable part of the bile that has been secreted and poured into the intestinal canal is destined for re-absorption. And if we admit that the spleen, thymus and thyroid bodies, and supra-renal capsules, are to be regarded as possessing a glandular character, although the products of their elaboration are destined to be received back again into the current of the circulation, it is difficult to find a reason for the exclusion of a mass of adipose tissue from the same category.