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 6th 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).

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,, breaks up into capillaries, ultimately forming the veins, g.

the body. They are fewest in the back and neck, where their number is not much more than 400 to a square inch. They are more numerous on the skin of the palm and sole, where their apertures follow the ridges visible on the skin, and amount to between two and three thousand on the square inch. At a rough estimate, the whole integument probably possesses not fewer than from two millions and a quarter to two millions and a half of these tubules, which therefore must possess a very great aggregate secreting power.

a

FIG. 33

Coiled end of a sweat-gland (Fig. 31, g), epithelium not shown. a, the coil; b, the duct; c, network of capillaries, inside which the duct gland lies.

17. The sweat-glands are greatly under the influence of the nervous system. This is proved, not merely by the well-known effects of mental emotion in sometimes suppressing the perspiration and sometimes causing it to be poured forth in immense abundance, but has been made a matter of direct experiment. There are some animals, such as the horse, which perspire very freely.

If the