of glandulæ in other parts of the skin is sometimes greater, but generally less than this; and, according to Mr. Wilson, about 2800 may be taken as the average number of pores in each square inch throughout the body. Now the number of square inches of surface, in a man of ordinary stature, is about 2500; the number of pores, therefore, is seven millions; and the number of inches of perspiratory tubing would thus be 1,750,000, or 145,833 feet, or 48,611 yards, or nearly 28 miles.

744. From this extensive system of glandulæ, a secretion of watery fluid is continually taking place; and a considerable amount of solid matter also is drawn off by the epithelium-cells that line the tubuli. Under ordinary circumstances, the fluid is carried off in the state of vapor, forming the insensible perspiration; and it is only when its amount is considerably increased, or when the surrounding air is already so loaded with moisture as to be incapable of receiving more, that the fluid remains in the form of sensible perspiration upon the surface of the skin. It is difficult to estimate the proportion of solid matter contained in this secretion; partly on account of the great variations in the amount of fluid eliminated by the Sudoriparous glands, which are governed by the temperature of the skin; and partly because the secretion can scarcely be collected for analysis free from the sebaceous and other matters which accumulate on the surface of the skin. According to Anselmino it varies from to 1 per cent.; and consists in part of lactic acid, to which the acid reaction and sour smell of the secretion are due; in part of a proteine-compound, which is probably furnished by the epithelium-cells that line the tubes; and in part of saline matters, directly proceeding from the serum of the blood. Urea has been recently detected in the perspiration of the inhabitants of warm climates.

745. The amount of fluid excreted from the skin is almost entirely dependent upon the temperature of the surrounding medium; being increased with its rise, and diminished with its fall. The object of this variation is very evident; being the regulation of the temperature of the body. When the surface is exposed to a high degree of external heat, the increased amount of fluid set free from the perspiratory glands becomes the means of keeping down its own temperature; for this fluid is then carried off in a state of vapor, as fast as it is set free; and in its change of form, it withdraws a large quantity of caloric from the surface. But if the hot atmosphere be already loaded with vapor, this cooling power cannot be exerted; the temperature of the body is raised, and death supervenes, if the experiment be long continued. The cause of the increased secretion is probably to be looked for in the increased determination of blood to the skin, which takes place under the stimulus of heat. The entire loss by Exhalation from the lungs and skin, during the twenty-four hours, seems to average a little above 2 lbs. In a warm dry atmosphere, however, it has been found to rise to as much as 5 lbs. whilst in a cold damp one, it may be lowered to 13lb. Of this quantity, the pulmonary exhalation is usually somewhat less than one-third, and the cutaneous somewhat more than two-thirds; but when the quantity of fluid lost is unusually great, the increase must be chiefly in the Cutaneous exhalation; since, as already pointed out (§ 701), the amount of

exhalation from the lungs is not influenced by the external temperature, but only by the degree in which the surrounding air is previously saturated with moisture.

746. The variations in the amount of fluid set free by Cutaneous and Pulmonary Exhalation, are counterbalanced by the regulating action of the Kidney; which allows a larger proportion of water to be strained off in a liquid state from the blood-vessels, as the Exhalation is less,—and vice versa. The Cutaneous and Urinary excretions seem to be vicarious, not merely in regard to the amount of fluid which they carry off from the blood, but also in respect to the solid matter which they eliminate from it. It appears that at least 100 grains of effete azotized matter are daily thrown off from the skin; and any cause which checks this excretion, must increase the labor of the Kidneys, or produce an accumulation of noxious matter in the blood. Hence attention to the functions of the skin, at all times a matter of great importance, is peculiarly required in the treatment of Urinary diseases; and it will be often found that no means is so useful in removing the lithic acid deposit, as copious ablution and friction of the skin, combined with exercise. When the Exhalent action of the skin is completely checked by the application of an impermeable varnish, the effect is not (as might be anticipated) an elevation of the temperature of the body; on the contrary it is lowered, in consequence, as it would appear, of the interruption to the aeration of the blood through the skin, which is a function of such importance in the lower animals (§ 671), and of no trifling account in Man; and in a short time, a fatal result ensues. A partial suppression by the same means gives rise to febrile symptoms, and to Albuminuria, or escape of the albuminous part of the liquor sanguinis into the urinary tubes, in consequence (it would appear) of the increased determination which then takes place towards the Kidneys. These facts are interesting, as throwing light upon the febrile disturbance which accompanies those cutaneous diseases that affect the whole surface of the skin at once, and interfere with its functions; and as partly accounting also for the Albuminuria which frequently manifests itself during their progress, especially in Scarlatina.

747. The Skin is likewise furnished with numerous Sebaceous glands, which are distributed more or less closely over the whole surface of the body; being least abundant where the Perspiratory glandulæ are most numerous; and vice versa. They are altogether absent on the palms of the hands and the soles of the feet; and are particularly frequent in the skin of the face and in the scalp. They differ greatly in size and in degree of complexity; sometimes consisting of short straight follicles; sometimes closely resembling the Sudoriparous glandulæ, the tubes, however, being usually straighter and wider; and being sometimes much more complex in structure, consisting of a number of distinct sacculi clustered around the extremity of a common duct, into which they open, and forming little arborescent masses about the size of millet-seeds. In some situations they acquire still greater complexity. Thus the Meibomian glandulæ, which are found at the edges of the eyelids, and which secrete an unctuous matter for their lubrication, are long sacculi branching out at the sides (Fig. 111); and the glandulæ of the ear passage, which secrete

its cerumen or waxy matter, and which belong to the general Sebaceous system, are formed of long tubes, highly contorted, and copiously supplied with blood-vessels. In the hairy parts of the skin, we usually find a pair of Sebaceous follicles opening into the passage through which every hair ascends (Fig. 130, 9). The purpose of the sebaceous secretion is evidently to prevent the skin from being dried and cracked by the influence of the sun and air. It is much more abundant in those races of mankind which are formed to exist in warm climates, than in the races that naturally inhabit cold countries; and the former are accustomed to aid its preservative power, by lubricating their skin with vegetable oils of various kinds; which process they find to be of use, in protecting it from the scorching influence of the solar rays.-The Sebaceous follicles are frequently the residence of a curious parasite, the Demodex folliculorum, which is stated by Mr. Erasmus Wilson to be present in great numbers in the skin of almost all inhabitants of large towns; the activity of their cutaneous glandular system being much checked by the want of free exposure to pure air, and by inert habits of

life.

748. To what extent the Sebaceous secretion can be regarded as destined to free the Blood from deleterious matters, it may not, perhaps, be very easy to say; but with regard to the functions of the Skin taken altogether, as a channel for the elimination of morbific matters from the blood, it is probable that they have been much underrated; and that much more use might be made of it in the treatment of diseases,especially of such as depend upon the presence of some morbific matter in the circulating current,-than is commonly thought advisable. We see that Nature frequently uses it for this purpose; a copious perspiration being often the turning-point or crisis of febrile diseases, removing the cause of the malady from the blood, and allowing the restorative powers free play. Again, certain forms of Rheumatism are charac terized by copious acid perspirations; and instead of endeavoring to check these, we should rather encourage them, as the best means of freeing the blood from its undue accumulation of lactic acid. And it is recorded that in the "sweating sickness," which spread throughout Europe in the 16th century, no remedies seemed of any avail but diaphoretics; which, aiding the powers of nature, concurred with them to purify the blood of its morbific matter. The hot-air bath, in some cases, and the wet sheet (which, as used by the Hydropathists, is one of the most powerful of all diaphoretics), will be probably employed more extensively as theurapeutic agents, in proportion as the importance of acting on the Skin, as an extensive collection of glandulæ, comes to be better understood. The absurdity of the "Hydropathic" treatment consists in its indiscriminate application to a great variety of diseases; no person who has watched its operation, can deny that it is a remedy of a most powerful kind; and if its agency be fairly tested, there is strong reason to believe, that it will be found to be the most valuable curative means we possess for various specific diseases, which depend upon the presence of a definite "materies morbi" in the blood, especially Gout and chronic Rheumatism; as well as for that depressed state of the

general system, which results from the "wear and tear" of the bodily and mental powers.

749. The Mucous surface of the Alimentary Canal is furnished, like the skin, with a vast number of glandulæ, varying in complexity, from the simple follicle, to a mass consisting of numerous lobules opening into a common excretory duct. The functions of these, as already pointed out, are equally various. The simple follicles appear destined, for the most part, to secrete the protective mucus, which intervenes between the membranous wall and the substances contained in the canal, and which serves to protect the former from the irritating action of the latter. The more complex follicles of the Stomach elaborate the Gastric fluid, which is the prime agent in the digestive process (§ 496). The still more elaborate glandule of Brunner, situated in the walls of the duodenum, also seem to furnish a product which is concerned in the digestive operation (§ 480). But there is strong reason to believe, that the function of the Peyerian glandulæ, which beset the walls of the lower part of the intestinal canal, is purely excretory; and that they are destined to eliminate putrescent matters from the blood, and to convey them, by the readiest channel, completely out of the body. That the putrescent elements of the fæces are not immediately derived from the food taken in, so much as from the secreting action of the intestinal glandulæ, appears from this consideration;-that fæcal matter is still discharged, even in considerable quantities, long after the intestinal tube has been completely emptied of its alimentary contents. We see this in the course of many diseases, when food is not taken for many days, during which time the bowels are completely emptied of their previous contents by repeated evacuations; and whatever then passes, must be derived from the intestinal walls themselves. Sometimes a copious flux of putrescent matter continues to take place spontaneously; whilst it is often produced by the agency of purgative medicine. The "colliquative diarrhoea," which frequently comes on at the close of exhausting diseases, and which usually precedes death by starvation, appears to depend, not so much upon a disordered state of the intestinal glandulæ, as upon the general disintegration of the solids of the body, which calls them into extraordinary activity, for the purpose of separating the decomposing matter.

750. Thus we perceive, that we have here, also, to watch for the indications of Nature; and that this extensive system of intestinal glandulæ, being the principal channel for the elimination of putrescent matters from the blood, should be especially attended to, when there is reason to think that such matters are present in too large an amount. Hence, when diarrhoea is already existing, we may often do more good by allowing it to take its course, or even by increasing it by the agency of purgative medicines, than by attempting to check it, and thus causing the retention of the morbid matter in the circulating current. But, on the other hand, it is necessary to bear in mind the extreme irritability of the intestinal mucous membrane; and carefully to avoid exciting it, when it is already in excess, or when there is danger that it will supervene,-as in that form of Fever in which there is a peculiar

liability to inflammation and ulceration of the walls of the alimentary canal, and of their contained glandulæ.

5. General Summary of the Excreting Processes.

751. We have now passed in review the various processes, by which the products of the disintegration of the animal tissues are carried off; and we have seen that the necessity for their removal is much more urgent than for their replacement. A cold-blooded animal may subsist for some weeks, or even months, without a fresh supply of food, the waste of its tissues being so small, if it remain in a state of rest, as to be quite compatible with the continuance of its life; and a warmblooded animal may live for many days or even weeks, provided that it has in its body a store of fat sufficient to keep up its heat by the combustive process. But in either case, if the exhalation of carbonic acid by the lungs, the elimination of biliary matter by the liver, the separation of urea or uric acid by the kidneys, or the withdrawal of putrescent matter by the intestinal glandulæ, be completely checked, a fatal result speedily ensues ;-more speedily in warm-blooded animals, than in those which cannot sustain a high independent temperature, on account of the greater proneness to decomposition in the bodies of the former, than in those of the latter;-and more speedily in the latter, when their bodies are kept at an elevated temperature by the warmth of the surrounding medium, that when the degree of heat is so low, that there is little proneness to spontaneous change in the substance of their bodies.

752. It may be taken as a general principle, in regard to the Excreting processes (including Respiration), that they have a threefold purpose; in the first place, to carry off the normal results of the waste or disintegration of the solid tissues, and of the decomposition of the fluids;-in the second place, to draw off the superfluous alimentary matter, which though received into the circulating current, is not converted into solid tissue, in consequence of the want of demand for it; -and in the third place, to carry off the abnormal products, which occasionally result from irregular or morbid changes in the system. Thus by the Lungs are excreted a large amount of carbon, and some hydrogen, resulting from the disintegration of the tissues, especially the nervous and muscular; the same elements, in animals that take in a large proportion of farinaceous or oleaginous aliment, may be derived immediately from the food, without any previous conversion into solid tissue; and there can be little doubt that the respiratory function is also an important means of purifying the blood from various deleterious matters, either introduced from without (such as narcotic poisons), or generated within the body (such as the poison of fever).* And it is important to bear this last circumstance in mind; since it enables us to understand how, if time be given, the system frees itself from such noxious substances; and points out the duty of the medical attendant to be rather that

* There is strong reason to believe that, in many instances, a small amount of poisonous matter introduced from without, in the form of a contagion or miasm, may lead, by a process resembling fermentation, to the production of a large quantity of similar noxious substances in the animal fluids.