EXCRETION OF SUPERFLUOUS AZOTIZED NUTRIMENT. 295 separate or strain-off, as it were, the products of the decomposition of the tissues formed from it, when their term of life had expired (§ 161). But it is certain that Man (as well as other animals which have in some degree learned his habits) frequently consumes much more food than is necessary for the supply of his wants; and a little consideration will show, that the surplus must pass-off by these excretions, without ever forming part of the living fabric. For new muscular tissue is not formed in proportion to the quantity of aliment supplied, but in proportion to the demand created by the exercise of it (§ 587); consequently, if more food be taken-in than is necessary to supply that demand, no use can be made of it. We never find that a Man becomes more fleshy by eating a great deal and taking little exercise; indeed, the very contrary result happens, his flesh giving place to fat. But let him put his muscles to regular and vigorous exercise, and eat as much as his appetite demands, and they will then increase both in strength and bulk. 348. Hence, if more azotized food be taken-in, than is required to supply the waste of the muscular and other azotized tissues, the surplus must be carried-off by the organs of excretion-chiefly, indeed almost entirely, by the Kidneys. By throwing upon them more than their proper duty, they become disordered and unable to perform their functions; hence the materials which they ought to separate from the blood accumulate in it, and give rise to various diseases of a more or less serious character, which might have been almost certainly prevented by due regulation of the diet. The most common of these diseases among the higher classes are Gout and Gravel; both of these may be often traced to the same cause, the accumulation in the blood of lithic acid, which results from the decomposition of the superfluous azotized food, and which the kidneys are not able to throw-off in the proper state, that is, dissolved in water. That these diseases are, comparatively speaking, rare among the lower classes, is at once accounted-for by the fact, that they do not take-in any superfluous azotized food;-all that they consume being appropriated to the maintenance of their tissues, and the kidneys having only to discharge their proper function of removing from the blood the products of the decomposition of these. 296 EXCRETION OF SUPERFLUOUS NON-AZOTIZED NUTRIMENT. 349. Hence the radical cure of these diseases, in most persons who have a sufficiently vigorous constitution and firm resolution to adopt it, is abstinence from all azotized nutriment-whether contained in animal flesh, bread, or other articles of vegetable diet, save such as is required to supply the wants of the system. If such abstinence be carried too far, however, it will produce injurious instead of beneficial results, weakening the fabric, and impairing the digestive powers; and if food be employed of a kind which is liable to produce lactic acid (the acid that appears in milk, when it turns sour), much disorder may still remain, which must be avoided by using the kind of diet that is least liable to undergo this change. 350. Again, if more non-azotized food is taken into the system than can be got rid of by Respiration, it must either be deposited as fat, or it must be separated from the blood, and carried-off by the excretion of the Liver. But if too much work be thrown upon this organ, its function becomes disordered, from its inability to separate from the blood all that it should draw-off; the injurious substances accumulate in the blood, therefore, producing various symptoms that are known under the general term of bilious; and to get rid of these, it becomes necessary to administer medicines (especially those of a mercurial character) which shall excite the liver to increased secretion. The constant use of these medicines has a very pernicious effect upon the constitution; and careful attention to the regulation of the diet, and especially the avoidance of a superfluity of oily or farinaceous matter, will generally answer the same end in a much better manner. 351. That the materials of the Biliary and Urinary excretions pre-exist (like the carbonic acid thrown-off by respiration) in the blood, in forms which, if not identical, are at any rate closely allied to those under which they present themselves in the bile and urine, has now been fully proved. The quantity of them present in the circulating fluid, however, is usually very small; for the simple and obvious reason that, if the excreting organs are in a state of healthy activity, these substances are drawn-off by them from the blood, as fast as they are introduced into it. But if the excretions be checked, they speedily accumulate in the blood, to such a degree as to be easily detected by the Chemist, and also to make their presence evident by their effects upon the animal functions, NATURE AND PURPOSES OF ANIMAL SECRETIONS. 297 This sometimes especially those of the nervous system. happens in consequence of disease, and it may be imitated by experiment; for when the trunk of the blood-vessel conveying the blood to the liver or kidney is tied, the excretion is necessarily checked, and the same results take place as when the stoppage has depended on want of secreting power. The biliary and urinary matters have the effect of narcotic poisons upon the brain; when they have accumulated in the blood, their symptoms begin to manifest themselves; and these symptoms increase in intensity, as the amount of the substances becomes augmented, until death takes place. 352. Besides the Excretions, we find various Secretions elaborated in different parts of the bodies of animals, with a view not so much to the purification of their blood, as to the fulfilment of special purposes in their economy. These vary considerably in the different classes of animals; though some of them, being concerned in functions almost universally performed, are equally general in their range. Thus we find the Salivary and Gastric fluids poured into the mouth and stomach, for the reduction and solution of the food (§§ 190 and 204); and the Lachrymal secretion poured out upon the surface of the eye, for the purpose of washing it from impurities (§ 541): while the secretion of Milk for the nourishment of the young is limited to Mammals; and poisonous secretions are formed in Serpents and Insects, for the destruction of their prey or for means of defence. Any one of these may be checked, without rendering the blood impure by the accumulation of any substances that should be drawn-off from it; but its cessation may produce effects fully as injurious, by disordering the function to which it is subservient. Thus, if the salivary and gastric secretions were to cease, the reduction of the food could not be effected, and the animal must starve, though its stomach were filled with wholesome aliment.-It is to be observed, in regard to nearly all these secreted fluids, that they contain but a small quantity of solid matter, and that this matter seems to be formed from the albumen of the blood by a process of incipient decomposition, which gives it the character of a "ferment." 353. The various acts of Secretion and Excretion which are continually taking place in the living body, are, like those of Nutrition, completely removed from the influence of the will; 298 INFLUENCE OF EMOTIONS UPON SECRETIONS. but they are strongly affected by emotions of the mind. This has been already pointed out in regard to the Saliva (§ 190); and it is equally evident in the case of the Lachrymal secretion (§ 541). In these instances, however, the effect of the emotion is manifested upon the quantity only of the secretion; in the case of the secretion of Milk, not only the quantity but quality is greatly influenced by the mental state of the nurse. The more even her temper, and the more free from anxiety her mind, the better adapted will be her milk for the nourishment of her offspring. There are several instances now on record, in which it has been clearly shown, that the influence of violent passions in the mother has been so strongly exerted upon the secretion of milk, as almost instantaneously to communicate to it an absolutely poisonous character, which has occasioned the immediate death of the child.1 The influence of emotional states upon the Secretions is probably communicated by the Sympathetic system of Nerves (§ 461), which is very minutely distributed, with the blood-vessels, to the several glands which form them. Nature of the Secreting Process.-Structure of the Secreting Organs. 354. Notwithstanding the different characters of the products of Secretion and Excretion, and the variety of the purposes to which they are destined to be applied, the mode in which they are elaborated or separated from the blood is essentially the same in all. The process is performed, in the Animal, as in the Plant, by the agency of cells; and the variety of the structure of the different Glands, or secreting organs, has reference merely to the manner in which these, their essential parts, are arranged. The simplest condition of a secreting cell, in the animal body, is that in which it exists in Adipose or fatty tissue; which is composed, as formerly explained (§ 46), of a mass of cells, bound together by areolar tissue that allows the blood-vessels to gain access to them. Every one of these cells has the power of secreting or separating fatty matter from the blood; and the secreted product remains stored-up in its cavity, as in Plants (VEGET. 1 See the Author's Principles of Human Physiology, chap. XV.; and Dr. A. Combe on the Management of Infancy, chap. x. ESSENTIAL STRUCTURE OF SECRETING ORGANS. 299 PHYS. § 324)-not being poured forth, as it is in most other cases, by the subsequent bursting of the cell. 355. But when the secreting cells are disposed on the surface of a membrane, instead of being aggregated in a mass, it is obvious that, if they burst or dissolve-away, their contents will be poured into the cavity bounded by that membrane; and this is the mode in which secretion ordinarily takes place. Thus, the Mucous Membranes (§ 39) are covered with epithelium-cells, which are continually being cast-off, and which are replaced as constantly by a fresh crop; and they form by their dissolution the glairy viscid substance termed mucus, which covers the whole surface of the membrane, and serves for its protection. In parts of the membrane where it is necessary that the secretion should be peculiarly abundant, we find its secreting surface greatly increased, by being prolonged into vast numbers of little pits or bags, termed follicles, which are lined with epithelium-cells, that resemble those of its general surface (see fig. 9). Such follicles are very abundant along the whole alimentary canal of Man; and the glandulæ in which the Gastric and Intestinal fluids are elaborated, are almost equally simple in their structure (§ 204). 356. Now although the most important Secretions and Excretions are separated, in Man and the higher animals, by organs of a much more complex nature, yet in the lower we find them generated after the same simple fashion. Thus in the little Bowerbankia (§ 115), the bile is secreted by minute follicles which are lodged in the walls of the stomach (fig. 64, c) and pour their secretion separately into its cavity, having no communication with one another. In more complex forms of glan Fig. 164.-GLAND THAT SECRETES THE dular structure, however, several follicles open together into a tube, which discharges the product of their secretion |