310 00 a STRUCTURE OF THE KIDNEY. a с d whilst the inner, b, is termed medullary (or pith-like). In the cortical substance, no definite arrangement can be detected by the naked eye; it chiefly consists of a very intricate network of blood-vessels, surrounding the extremities of the tubes. But in the medullary substance we can trace a regular passage of minute tubes, from the circumference towards the centre. They commence in the midst of the network of blood-vessels (B, a), and then pass down in clusters, nearly in a straight direction, and slightly converging towards each other, until each cluster terminates in a little body, called the calyx or cup, which discharges the fluid it receives into the large cavity of the kidney, termed the pelvis or basin (A, c). From this it is conveyed away by the ureter d, which terminates in the bladder. C B Fig. 172.-STRUCTURE OF THE A, vertical section of the kidney; substance; c, calyx and pelvis ; d, ureter. B, portion of the gland enlarged; a, extremity of the uriniferous tubes; b, straight portion; c, their termination in the calyx. 369. One of the most interesting circumstances in reference to the Urinary secretion, is the very large quantity of water which, in the higher animals, is got rid of through this channel, and the means by which it is drawn off. The kidneys seem to form a kind of regulating valve, by which the quantity of water in the system is kept to its proper amount. The exhalation from the Skin is liable to sustain great variations in its amount from the temperature of the air around; for when this is low, the exhalation is very much diminished; and when it is high, the quantity of fluid that passes off in this manner is increased (§ 371). Hence, if there were not some other means of adjusting the quantity of fluid in the blood-vessels, it would be liable to continual and very injurious variation. This important function is performed by the kidneys, which allow such a quantity of water to pass into MALPIGHIAN BODIES OF THE KIDNEY, 311 the urinary tubes, as may keep the pressure within the vessels very nearly at a uniform standard; an a distinct and very curious provision exists for its separation. The extremity of many of the uriniferous tubes is made to include little knots or bunches of capillary vessels, which have extremely thin walls (fig. 173); and a vast number of such knots, which are termed "Malpighian bodies," after the name of their discoverer, are scattered through the cortical portion (§ 368) of the kidney. To these the blood brought to the organ by the renal artery is first conveyed; and the membranes that separate the interior of the capillary vessels from the cavity of the uriniferous tube, being of Fig. 173.-MALPIGHIAN BODIES OF extreme thinness, water is THE KIDNEY. readily able to traverse them; and. will do so in larger or smaller quantity, according as the pressure upon the walls of the capillaries is greater or less. The blood which has passed through these is next conducted to another set of capillaries, which form a network upon the part of the tube that is lined by the secreting cells; and it is there subservient to the elaboration of the solid part of the secretion. Hence the quantity of water in the urinary secretion depends in part upon the amount exhaled from the skin,-being greatest when this is least, and vice versa,-and in part upon the quantity which has been absorbed by the vessels. The quantity of solid matter in the secretion has but little to do with this; for it depends upon the amount of waste of the muscular and other tissues that has been occasioned by their activity (§ 367); and also upon the quantity of surplus aliment which has to be discharged through this channel, there being no other vent for it (§ 348). 370. Next to the excretions formed by the liver and the kidneys, that of the Skin probably ranks in importance. A large quantity of watery vapour is constantly passing-off from the whole surface of Man and other soft-skinned animals; 312 EXHALATION FROM THE SKIN. and this amount is greatly increased under particular circumstances. A continual evaporation takes place from the surface of the skin, wherever it is not protected by hard scales or plates; and the amount of it will depend upon the warmth, dryness, and motion of the surrounding air, exactly as if the fluid were being evaporated from a damp cloth. Every one knows that the drying of a cloth is much more rapidly effected in a warm dry atmosphere, than in a cold moist one; more quickly, too, in a draught of air, than in a situation where there is no current, and where the air is consequently soon charged with moisture. That all these influences affect the evaporation from the bodies of Animals, there is ample evidence derived from experiment. 371. But besides this continual evaporation, a special exhalation of fluid takes place from the vast number of minute perspiratory glands imbedded in the fatty layer just beneath the Skin (§ 37). Every one of these glandulæ, when straightened out, forms a tubule about a quarter of an inch in length; and as it has been estimated that in a square inch of surface on the palm of the hand there are no fewer than 3528 of these glandulæ, the length of their tubing must be 882 inches or 731 feet. The average number in other parts of the body may be estimated at about 2800 per square inch; and as the number of square inches of surface on a man of ordinary stature is about 2500, the total number of perspiratory glandulæ must be not less than seven millions, and the length of their tubing nearly twenty-eight miles. The fluid which these perspiratory glands ordinarily exhale, is dissolved by the atmosphere, and carried off in the state of vapour, so as to pass away insensibly; but they are stimulated to increased action by the exposure of the body to heat, which causes them to pour forth their secretion in greater abundance than the air can carry off, and this consequently accumulates in drops upon the surface of the skin. The amount of perspiration may be considerably increased, without its becoming sensible, if the air be warm and dry, so as to carry off, in the form of vapour, the fluid which is poured out on the skin; but, on the other hand, a very slight increase in the ordinary amount immediately becomes sensible on a damp day, the air being already too much loaded with moisture to carry off this additional quantity. The distinction between insensible and sensible COOLING EFFECT OF CUTANEOUS EXHALATION. 313 perspiration, is not the same, therefore, with the difference between simple evaporation and exhalation from the skin; for a part of the latter is commonly insensible; and the degree in which it is so depends upon the amount of fluid exhaled, and the state of the surrounding atmosphere. If the fluid thus poured forth be allowed to remain upon the surface of the skin, it produces a very oppressing effect; most persons have experienced this, when walking in a mackintosh cloak or coat, on a damp day. The waterproof garment keeps in the perspiration, almost as effectually as it keeps out the rain; and consequently the air within it becomes loaded with fluid, and the skin remains in a most uncomfortable as well as prejudicial state of dampness. 372. The purpose of this watery exhalation, and of its increase under a high temperature, is evidently to keep the heat of the body as near as possible to a uniform standard. By the evaporation of fluid from the surface of the skin, a considerable quantity of heat is withdrawn from it, becoming latent (according to ordinary phraseology) in the change from fluid to vapour: of this we make use in applying cooling lotions to inflamed parts. The more rapid the evaporation, the greater is the amount of heat withdrawn in a given time; hence, if we pour, on separate parts of the back of the hand, small quantities of ether, alcohol, and water, we shall find that the spot from which the ether is evaporating feels the coldest, that which was covered by the alcohol less so, whilst the part moistened with water is comparatively but little chilled. The greater the amount of heat applied to the body, then, the more fluid is poured out by the perspiratory glands; and as the air can carry it off more readily in proportion to its own heat, the evaporation becomes more rapid, and its cooling effect more powerful. It is in this manner that the body is rendered capable of sustaining very high degrees of external heat, without suffering injury. Many instances are on record, of a heat of from 250° to 280° being endured in dry air for a considerable length of time, even by persons unaccustomed to a peculiarly high temperature; and individuals whose occupations are such as to require it, can sustain a much higher degree of heat, though perhaps not for any great length of time. Thus, the workmen of the late Sir F. Chantrey were accustomed to enter a furnace in which his 314 IMPORTANCE OF CUTANEOUS EXHALATION. moulds were dried, while the floor was red-hot, and a thermometer in the air stood at 350°; and Chabert, the "Fireking," was in the habit of entering an oven whose temperature was from 400° to 600°. It is possible that these feats might be easily matched by many workmen, who are habitually exposed to high temperatures; such as those employed in iron-foundries, glass-houses, and gas-works. 373. That the power of sustaining a high temperature mainly depends upon the dryness of the atmosphere, is evident from what has just been stated; since, if the perspiration that is poured-forth upon the skin is not carried-off with sufficient rapidity, on account of the previous humidity of the air, the temperature of the body will not be sufficiently kept down. It has been found, from a considerable number of experiments, that when warm-blooded animals are placed in a hot atmosphere saturated with moisture, the temperature of their bodies is gradually raised 12° or 13° above the natural standard; and that the consequence is then inevitably fatal. 374. The amount of fluid exhaled from the skin and lungs (§ 343) in twenty-four hours, probably averages about three or four pounds. The largest quantity ever noticed, except under extraordinary circumstances, was 5 lbs. ; and the smallest, 18 lbs. It contains a small quantity of solid animal matter, besides that of the other secretions of the skin which are mingled with it; and there is good reason to think that this excretion is of much importance, in carrying off certain substances which would be injurious if allowed to remain in the blood. That which is called the Hydrophatic system, proceeds upon the plan of increasing the cutaneous exhalation to a very large amount; and there seems much evidence, that certain deleterious matters, the presence of which in the blood gives rise to Gout, Rheumatism, &c., are drawn off from it more speedily and certainly in this way, than in any other. 375. Besides the perspiratory glands, the skin contains others, which have special functions to perform. Thus in most parts which are liable to rub against each other, we find a considerable number of sebaceous follicles (fig. 8, a a), which secrete a fatty substance that keeps the skin soft and smooth. Besides these, the skin contains other follicles in particular parts, for secreting peculiar substances; as, for instance, those which form the cerumen, a bitter waxy substance that is |