732. When the secreting apparatus consists of simple cryptæ, follicles, cæca, or tubuli, a similar net-work of capillary arteries and nerves is spread out on the sides of this more extended surface. The more elaborate secretion now formed is received into the interior of these organs, where it remains for some time, and whence it is ultimately conveyed as it is needed by the actions of the system.
733. But when the secreting apparatus consists of aggregates of cryptæ, follicles, cæca, and tubuli, with their net-works of arteries and nerves, a much more complex structure is built up, which is destined to perform a proportionably elaborate function. An aggregation of these secreting bodies into a large mass, enveloped in a common membrane, so as to form a distinct body of a solid consistence, constitutes the organ termed a gland. Simply extended membrane, with its apparatus of arteries and nerves does not constitute a gland. Simple cryptæ, follicles, cæca, and tubuli, with their larger apparatus of arteries and nerves, do not constitute a gland. The first is simply secreting surface; the second are simply secreting cryptæ, follicles, cæca or tubuli; but when these bodies are aggregated into dense and solid masses with an extended system of excretory ducts, and when the whole of this apparatus is inclosed in a proper membrane so as to form a distinct body, such a body is termed a gland.
734. Primary aggregations of these secreting bodies constitute what is termed a conglobate, that is, a simple gland; such are all the glands connected with the absorbent or lymphatic system. Secondary aggregates, or aggregates composed of simple glands, constitute what is termed a conglomerate, that is, a compound gland; such are all the organs commonly termed viscera, as the liver, the spleen, the pancreas, the kidney, and so on.
735. The conglobate, or simple gland, being formed by the aggregation of cryptæ, follicles, cæca, or tubuli, inclosed in a proper membrane, presents the appearance of a simple solid body, commonly of a rounded or oblong form (fig. [CLXXVI]. 516). On the contrary, the conglomerate or compound gland, being formed by the aggregation of conglobate or simple glands, presents the appearance of a compound body composed of a congeries of masses (fig. [CLXV]. 1). The larger masses enveloped in their own proper membrane are termed lobes (fig. [CXCI].); the smaller masses, also enveloped in their own proper membrane, are termed lobules (fig. [CXCI].); the lobules, when carefully examined, are seen to be composed of still smaller masses, and these of masses yet more minute, until at length patient, laborious, and skilful dissection brings into view the ultimate constituent elements, which are invariably found to consist of simple cryptæ, follicles, cæca, or tubuli.
736. Thus membrane having a specific arrangement of blood-vessels and nerves, from being simply extended, is folded into a few elementary forms; the bodies which result constitute simple secreting organs; these bodies collected together form, by their aggregation, compound organs; the compound organs, uniting, form aggregates still more compound, until at length a structure is built up highly elaborate and complex. But this complexity of combination and arrangement does not alter the constitution of the organs; their form varies, but their nature remains essentially the same. All consist alike of membrane organized in a similar mode. The complex contains no element not possessed by the simple gland, and the gland contains no element not possessed by the secreting surface. But there is this difference in the complex organs. Every kind and degree of change in the form of the secreting apparatus, from membrane simply extended, to membrane coiled up into the most complex gland, is attended with an accumulation and concentration of secreting surface. The crypt contains a larger extent of secreting surface than the simple membrane; the follicle than the crypt; the cæcum than the follicle; and the tubulum than the cæcum. A certain amount of secreting surface is gained by the disposition of the simple membrane into the form of the crypt. The collection of a number of crypts into a cluster doubles the extent of the secreting surface by the extent of every crypt that is added to the cluster. The addition of every cluster doubles the whole extent of surface acquired by a single cluster. But when stems spring as if from a common trunk; when branches spring from a stem; when small branches spring from the large branches, and yet smaller branches from the small in a series, which the eye, assisted by the most powerful microscope, is wholly unable to trace; when all the clusters thus formed are collected, and combined into a compact mass, the intricacy of which no art can completely unravel, the extent of surface obtained is altogether immeasurable. How immense must be the extent of surface thus acquired in such an organ as the human lungs, in such a gland as the human liver!
737. In such an aggregation the concentration is also equal to the accumulation; the maximum of surface is comprised in the minimum of space, and the energy and elaborateness of the function of a secreting organ is uniformly proportionate to such a concentration of its secreting substance.
Aggregated and clustered cæca opening into the alimentary canal, performing the function of the liver.
738. Hence the complexity of the compound gland in the higher animals would appear to arise solely from the intricate arrangement of the immense mass of secreting matter concentrated in a small compass; hence also the progressively increased complication indicated in the successive development of the glandular system in the animal series. Thus, for example, among the distinct organs formed for the purpose of elaborating a specific secretion, being intimately connected with the process of digestion, one of the first is the salivary gland. Low down in the scale, in the animal in which the first rudiment of a salivary gland is traceable, it consists of a single follicle, which appears to serve the office of a gland. In an animal a little higher in structure, two, three, or four follicles combine to form a somewhat less simple organ. In an animal still higher in the series, a number of follicles are clustered together and form a much more complex organ; and in this manner, as the organization of the animal becomes higher and higher, the complexity of the gland increases, until at length it is composed of a countless number of follicles collected into clusters, the clusters disposed into lobes, the lobes subdivided into lobules, and the lobules into still smaller particles, the ultimate elements of the glandular apparatus. In like manner, when the first rudiment of the liver is discoverable, it consists of a single pouch or cæcum; somewhat higher in the series, the organ is composed of two or more cæca distinct and free; and then, as its complexity increases with the perfection of the organization, cæca are accumulated upon cæca; the aggregates so formed are closely compacted, disposed into lobes, divided into lobules, and subdivided into the ultimate particles of the glandular apparatus. So in a gland composed of tubuli, as the kidney, the organ in its rudimentary state consists of a few straight tubuli: as its structure advances more tubuli are added: next, the increasing tubuli superimposed one upon another become tortuous; then the tubuli still accumulating, become not merely tortuous, but convoluted; and last of all, countless numbers of tubuli are closely compacted into exceedingly convoluted masses. Uniformly, the lower the animal and the simpler the organ, the larger and the more manifest are the elementary parts of the gland; but in the higher animals these elementary bodies are so minute as to be altogether microscopical and their arrangement is so complex that it can be unravelled only with extreme difficulty.