of blood the more rapid will be the escape of urine. Hence we find that when blood flows freely to the kidney the urine is secreted freely, but that when the blood supply to the kidney is scanty the urine also is scanty. When certain nerves going to the kidney are cut, the ramifications of the renal artery dilate, much blood goes into the kidney and the flow of urine is copious. If the same nerves be irritated, the arterial tubes are narrowed or constricted, less blood goes to the kidney, and the flow of urine is scanty or may be stopped altogether.

And this explains, in part at all events, how it is that the activity of the kidney is influenced by the state of the skin. The quantity of blood in the body, being about the same at all times, if a large quantity goes to the skin, as in warm weather and especially when the skin is active and perspiring, less will go to the kidney, and the secretion of urine will be small. On the other hand, if the blood be largely cut off from the skin, as in cold weather, more blood will be thrown upon the kidney and more urine will be secreted. Thus the skin and the kidneys play into each other's hands in their efforts to get rid of the superfluous water of the body.

12. But the whole of the urine is thus not secreted, through a sort of filtering process, by the Malpighian capsules. The tubules are lined, as has been stated, by epithelium cells, and these cells, in certain parts of the tubule, especially where these are coiled, are what is called secreting cells. That is to say they have the power, by some means which we do not at present fully understand, to take up from the blood, which is flowing in the capillaries wound round the tubules, or rather from the plasma which exudes from those capillaries and bathes the bases of the cells, certain substances, and to pour these substances, in some cases greatly changed, in some cases hardly or not at all changed, into the cavity of the tubule. As has been said, even the blood which escapes from the glomerulus and has therefore parted with some of the substances which go to form the urine, is carried to the capillary network wrapped round the tubules, and is there exposed to the further action of the epithelium cells which line those tubules, the plasma which exudes from the capillaries acting as a middle man between the

blood inside the capillary walls and the substance of the cells themselves.

And we have evidence that many of the most important constituents of the urine, such as urea, uric acid and others, are thus secreted by the epithelium cells of the tubules, and not simply filtered off by the Malpighian capsules.

The formation of urine is therefore a double process. A great deal of the water, with probably some of the more soluble inorganic salts, pass by the glomeruli, but the urea, the colouring matters and a great many other of the constituents, are thrown into the cavities of the tubules by a peculiar action of the epithelium cells, some of those substances being actually manufactured by the cell and not existing as such in the blood.

13. That the skin is a source of continual loss to the blood may be proved in various ways. If the whole body of a man, or one of his limbs, be enclosed in a caoutchouc bag, full of air, it will be found that this air undergoes changes which are similar in kind to those which take place in the air which is inspired into the lungs. That is to say, the air loses oxygen and gains carbonic acid; it also receives a great quantity of watery vapour, which condenses upon the sides of the bag, and may be drawn off by a properly disposed pipe.

Under ordinary circumstances no liquid water appears upon the surface of the integument, and the whole process receives the name of the insensible perspiration. But, when violent exercise is taken, or under some kinds of mental emotion, or when the body is exposed to a hot and moist atmosphere, the perspiration becomes sensible; that is, appears in the form of scattered drops upon the surface.

14. The quantity of sweat, or sensible perspiration, and also the total amount of both sensible and insensible perspiration, vary immensely, according to the temperature and other conditions of the air, and according to the state of the blood and of the nervous system. It is estimated that, as a general rule, the quantity of water excreted by the skin is about double that given out by the lungs in the same time. The quantity of carbonic acid is not above 3th or 4th of that excreted by the lungs; and it is not

certain that in health any appreciable quantity of urea is given off.

In its normal state the sweat, as poured out from the proper sweat-glands, is alkaline; but ordinarily, as it collects upon the skin it is mixed with the fatty secretion of the sebaceous glands, and then is frequently acid. In addition it contains scales of the external layers of the epidermis, which are constantly being shed.

15. In analysing the process by which the perspiration is eliminated from the body, it must be recollected, in the first place, that the skin, even if there were no glandular structures connected with it, would be in the position of a moderately thick, permeable membrane, interposed between a hot fluid, the blood, and the atmosphere. Even in hot climates the air is, usually, far from being completely saturated with watery vapour, and in temperate climates it ceases to be so saturated the moment it comes into contact with the skin, the temperature of which is, ordinarily, twenty or thirty degrees above its own.

A bladder exhibits no sensible pores; but if a bladder be filled with water and suspended in the air, the water will gradually ooze through the walls of the bladder, and disappear by evaporation. Now, in its relation to the blood, the skin is such a bladder full of hot fluid.

Thus, perspiration, to a certain amount, must always be going on through the substance of the integument, but probably not to any great extent; though what the amount of this perspiration may be cannot be accurately ascertained, because a second and very important source of the perspiration is to be found in what are called the sweat-glands.

16. All over the body the integument presents minute apertures, the ends of channels excavated in the epidermis or scarf-skin, and each continuing the direction of a minute tube, usually about th of an inch in diameter, and a quarter of an inch long, which is imbedded in the dermis. Each tube is lined with an epithelium continuous with the epidermis (Fig. 32, e). The tube sometimes divides, but, whether single or branched, its inner end or ends are blind, and coiled up into a sort of knot, interlaced with a meshwork of capillaries (Fig. 31, Ag, and Fig. 33).

1

The blood in these capillaries is therefore separated from the cavity of the sweat-gland only by the thin walls of the capillaries, that of the glandular tube, and its epithelium, which, taken together, constitute but a very thin pellicle; and the arrangement, though different in detail, is similar in principle to that which obtains in the kidney. In the latter, the vessel makes a coil within the Malpighian capsule, which ends a tubule. Here the perspiratory tubule coils about, and among, the vessels. In both cases the same result is arrived at-namely, the

A. Section of the skin showing the sweat-glands. a, the epidermis; b, its deeper layer, the rete Malpighii; e,d, the dermis or true skin;f, fat cells; g, the coiled end of a sweat-gland; h, its duct; i, its opening on the surface of the epidermis.

B. Section of the skin showing the roots of the hairs and the sebaceous glands. b, muscle of c, the hair sheath, on the left hand.

exposure of the blood to a large, relatively free, surface, on to which certain of its contents transude. In the sweat-gland however there is no filtering apparatus like the Malpighian corpuscle of the kidney, and the whole of the sweat appears to be secreted into the interior of the tube by the action of the epithelium cells which line it. The number of these glands varies in different parts of

Portion of Fig. 31 A, more highly magnified-somewhat diagrammatic. a, horny epidermis; b, softer layer, rete Malpighii; c, dermis; d, lowermost vertical layer of epidermic cells; e, cells lining the sweat duct continuous with epidermic cells; h, corkscrew canal of sweat duct. To the right of the sweat duct the dermis is raised into a papilla, in which the small artery, f, breaks up into capillaries, ultimately forming the veins, g.