These very various products are sepa rated from the blood by very different organs. The most simple mode of accre tion is that performed by the arteries of a part without any glandular apparatus ; as the fluids of circumscribed cavities, the lymph of the cellular substance, and the fat and marrow.
Secretion is more complicated when performed by means of certain organs called glands. '1'lle most simple of these are the mucous follicles, found in various parts of the alimentary and respiratory canals ; consisting of a small bag receiv ing the secreted fluid front the arteries, and expelling it through a short excreto ry duct. But the name of gland is applied more properly to the larger organs of complicated structure, as the pancreas, breast, salivary glands, &c. These, con sisting of an aggregation of minute parti cles, are called conglomerate, to distin guish them from the lymphatic or con globate glands. Each of them possesses an excretory duct, made up by the union of branches from the various component portions of the gland. The larger tons, into which each gland is resolved, may be divided into smaller and smaller particles, and ultimately into very minute portions ; concerning the structure of which, anatomists have warmly disputed. Some describe them as being hollow, and called theni acini, or crypts; while others asserted that they consist merely of convoluted blood-vessels : the latter opinion is the most prevalent at present. The structure of the liver and kidney is analogous to this, in its minute parts : both these organs, and particularly the latter, exhibit the acinous appearance. The ultimate blood-vessels are arranged in very different ways in various glands ; coiled up in roundish masses, as in the kidney, arranged like stars in the liver, and forming an appearance like a camel's hair pencil in the spleen.
The various properties of secreted flu ids depend, no doubt, more on the inte rior texture and vital powers of the se creting organs, than on their external ha ]iterated, and their place by others.
When the nutritive matter has been duly assimilated, the parts which it sup plies retain it, and incorporate it with their own substance. This nutritive ap propriation is variously effected in differ ent structures ; since each part. converts to its own use, by a true secretion, that which is found analogous to its nature, and rejects the heterogeneous particles.
The mechanism of nutrition would be explained, if we could understand how each function divests the aliments on which we exist of their characters, to in vest them with the properties of our or gans ; how each individual part- co-ope rates in changing their nutritious princi ple into our own peculiar structure. Ve getables, which form the sole nourish ment of man in many instances, and a ve ry great share of it in all cases, consist chiefly of carbon, hydrogen, and oxygen, with sometimes a small quantity of azote and salts. In the organs of the man fed osr these vegetables, azote predominates, and many new products are discoyered,. not distinguishable in the aliment, and, therefore, formed in the act of nutrition. Every living body, without exception, possesses this faculty of and de composing substances, and of giving rise to new products.
The marine plant, whose ashes form so da, if sown in a box filled with earth that does not contain a particle of that alkali, and moistened with distilled water, fur nishes it in as great a quantity as if the plant had been growing on the borders of the sea, and always supplied with salt water. Living bodies then are elaborato ries, in which such combinations and de compositions occur, as art cannot imitate ; bodies that to us appear simple, as soda and silex, seem to form themselves of other parts ; while some, whose compo sition we cannot determine, as certain metals, suffer inevitable decompositions; from which we may fairly conclude, that the pOwers of nature in the composition and decomposition of bodies far surpass the science of chemists, bit and conformation. For comparative anatomy shows us instances of the same fluid secreted by organs of very different obvious structure.
How, or why, certain organs secrete certain liquors, is the most important and essential question in this subject ; but one to which our ignorance will not enable us to reply in a satisfactory way. Proba bly the chief and proximate cause con sists in difference of structure, and per haps in the arrangement of the minute vessels, which are the organs of secretion. The peculiar powers of each part, its share of irritability, and contractility, must also have an important influence. The mechanical explanation of the phe nomenon, by the straining of the fluids through different sized pores, cannot be admitted for a moment. We have one fluid, the blood, sent into different or gans ; each of which separates from it a different produce of matter, differing in many instances from any contained be fore in the blood. Here then must be a decomposition and a recombination of elements produced by the living action of the gland.
Nutrition may be considered as the completion of the assimilating functions ; to which the processes hitherto describ ed, under the heads of Digestion, Ab sorption, Circulation, Respiration, and Secretion, are only preliminary and pre paratory. The food, changed in the man ner we have already described, anamalised and rendered similar to the being which it is designed to nourish, applies itself to those organs whose losses it is to supply, and this identification of nutritive matter to our organs constitutes nutrition. The component parts of the living body are incessantly carried off by various causes. Thus the machine is continually destroy ed, and at distant periods of life does not contain any of its original elements. Mad der, mixed with the food, dyes the bones of a red colour, which disappears when the use of the root is suspended. These phenomena can only be explained by ad mitting an entire removal and renewal of the bony particles. Now if the most com pact and solid parts be in a continual mo tion of decomposition and recomposition, this motion must be more rapid where the constituent principles are in the smallest degree of cohesion, as in fluids. l'hy,siologists have endeavoured to deter mine the period of the entire renovation of the body, and have considered that an interval of seven years is necessary for the original particles to be totally oh