" 4th. The wall, or genninal membrane, of the follicle is also (probably) in a state of progressive growth, acquiring additions to its length at the blind extremity, and becoming absorbed at its attached extremity. A pro gressive growth of this kind would account for the steady advance of its attached con tents, and would also place the wall of the follicle in the same category with the primary vesicle, gerniinal membrane, or wall of the acinus, in the vesicular glands.
" 5th. The primary secreting cells of the follicle are not always isolated. They are sometimes arranged in groups ; and when they are so, each group is enclosed within its parent-cell, the group of cells advancing in de velopment according to its position in the follicle, but never exceeding a particular size in each follicle."* Prof. Goodsir further expresses the opi nion, that there is an order of glands with very much elongated ducts, which do not possess " germinal spots" in particular situ ations, but in which these spots are diffused more uniformly over the whole internal sur face of the tubes. To this order he refers the kidney in Man and the higher Vertebrata.
We have thought it right to give Prof. Goodsir's statements in full, as being in the main unquestionably correct ; but we must express our own doubts as to that part of the doctrine which relates to the production of the secreting cells from distinct " germinal centres." We have examined a great number of membranes bearing an epithelial covering, without being able to discern these; and our own impression is, that the membrane itself is in a continual state of change, deriving from the blood-vessel, on the one side, the elements of the new growth, and yielding these up on the other.
Of the first development of secreting struc tures, a partial account was formerly given (see GLAND) ; and as this is on the whole in conformity with our present views, it is only requisite to add here the principal facts, as certained by microscopic research since that article was written.
The "plastic mass " of which the entire gland consists in its early condition, is now known to be composed of nucleated cells, which appear to be the parent-cells within which the true secreting cells are afterwards to be formed ; these parent-cells themselves becoming the vesicles or follicles of the gland, by the establishment of communications be tween their cavities and the branches of the duct. It seems probable that some of the original component cells of the gland coalesce or break down altogether, so as to form the smaller ducts, the development of which has been observed to be quite independent of the protrusion of the principal duct, and of its primary branches, from the cavity of which it is a diverticulum. These last are properly intercellular passages ; which, as Prof. Good sir justly observes, " is an important consider ation, inastnuch as it ranges them -in the same category with the intercellular passages and secreting receptacles of vegetables." Sources of the demand for the secreting function. — We must now consider in more detail, the causes which render the perform ance of this function essential to the active existence of every living being.
1. In the first place, nearly all the solids and fluids of the animal body are liable to continual decomposition and decay, in virtue of their peculiar chemical composition. That the living state antagonises this decay, and that decomposition can only take place after death, is a doctrine which long held undis puted sway in physiological science, but which is now generally admitted to be com pletely untenable. The resistance to decay which living organised structures present, is rather apparent than real; for it only con tinues so long as the circulating current con tinues to pass through or near them, carrying off the products of incipient decomposition, and replacing these by matter that is newly organised.
2. In the second place, a continual decom position and decay of an organised fabric is involved in its mere vegetative existence. For every portion of it has an individual and independent life, and a limited duration of its own : each part, like the simple isolated cell of the lowest Cryptogamia, grows from a germ, arrives at maturity, and finally dies and decays ; its debris being directly cast off, if the organ be external ; but being taken again into the current of the circulation, to be eliminated by another channel, if the part have no direct communication with the surface of the body.*
Perhaps the most obvious example of this general fact is presented to us in the vegetable economy. The cells of the woody stem have a long and almost indefinite duration, espe cially after they have become consolidated by the filling-up of their cavities with resinous or sclerogenous secretions ; but those of the leaves, which are much more actively con cerned in the vital operations, have a short and limited term of existence. The " fall of the leaf" is not the cause of the death and decay of the organ, but its result ; for the de composition of its tissues is already far ad vanced, when its detachment occurs : — its functions have been fulfilled ; its term of life is expired ; and it is cast off, to be replaced by a new development of cellular parenchyma, which in its turn will discharge the same im portant function, that of preparing the ma terials for the growth of the more permanent parts of the fabric. This kind of passive change is more constantly going on in the animal body than is usually supposed, espe cially during the period of its growth and in crease. A good illustration is afforded by the deciduous or milk teeth. " We trace each of these developed from its germ, and, in the course of its own development, separating a portion of itself to be the germ of its suc cessor ; then each, having attained its due perfection, retains for a time its perfect state, and still lives though it does not grow. But at length, coincidently, not consequently, as the new tooth comes, the deciduous tooth dies ; or rather, its crown dies, and is cast out like a dead hair ; while its fang with the bony sheathing, and the vascular and nervous pulp, degenerates, and is absorbed. It is here especially to be observed, that the de generation is accompanied by some spon taneous decomposition of the fang, for it could not be absorbed unless it was first so changed as to be soluble. And it is degeneration, not death, which precedes its removal; for when a tooth fang really dies, as that of the second tooth does in old age, then it is not absorbed, but is cast out entire, as a dead part. Such, or nearly such, it seems almost certain, is the process of assimilation every where; these may be taken as types of what occurs in other parts, for these are parts of complex organic structure and composition ; and the teeth-pulps, which are absorbed as well as the fangs, are very vascular and sensi tive, and therefore, we may be nearly sure, are subject to only the same laws as prevail in all equally organised parts."# All the epidermic and epithelial structures, including the secre tory substance of glands, are continually un dergoing the same change, by the exiiviation of the old cells when their term of life is ac complished, and by the production of new ones; the durability being different according to the particular endowments of the part, but also varying with changes in the supply of blood, which increase or decrease its vital activity. Generally speaking, those parts which live most slowly are those of which the duration is the greatest, and in which there ir con sequently the least frequent change. Of the exuviation of epidermic structures en masse — a process altogether comparable to the fall of the leaf — we have striking examples in the entire desquamation of serpents, the moulting of the plumage in birds, and the shedding of the hair in the mammalia ; and in the sheddino of the antlers of the stag, we have an example of the exuviation of a highly organised and vascular part, which periodically dies, and which, being external, is cast off entire. " What means all this," says Mr. Paget 4, " but that these organs have their severally appointed times, degenerate, die, are cast away, and in due time are replaced by others, which in their turn are to be developed to perfection, to live their life in the mature state, and in their turn to be cast off?" There can be little doubt that a similar change is continually taking place, with more or less activity, in every part of the internal struc ture ; the products of decay, however, not being at once thrown off, because there is no direct means of getting rid of them ; but being received back into the current of the circula tion, to be eliminated by instruments ex pressly provided for that purpose.