PHYSIOLOGY 



which a current of air is passed, and the temperature of the air gradually 

 raised, it will be noted that the amount of water given off rises slowly up 

 to a certain degree and then rises rapidly. The sudden kink in the curve is 

 due to the setting in of the activity of the sweat-glands, and we are there- 

 fore justified in regarding the insensible perspiration as being determined 

 by evaporation of water from the surface of the cuticle itself apart altogether 

 from the sweat-glands. The sweat-glands, which are distributed over the 

 whole surface of the skin, are especially abundant on the palm of the hand 

 and on the sole of the foot. They are composed of single unbranched 

 coiled tubes, which lie in the subcutaneous tissue and send their ducts up 

 through the cutis, to open on the surface by corkscrew-like channels which 

 pierce the epidermis. The secreting part of the tube consists of a basement 

 membrane lined by a double layer of cells ; the innermost of these are 

 cubical and represent the secreting cells proper. Between the secreting 

 cells and the basement membrane is a layer of unstriated muscle fibres. 

 The duct of the gland has an epithelium, consisting of two or three layers of 

 cells with a well-marked internal cuticular lining, but there is no muscular 

 layer. 



The sweat formed by these glands is the most dilute of all animal fluids. 

 As collected it generally contains epithelial scales and some admixture of 

 sebum. After filtration it forms a clear colourless fluid of a specific gravity 

 of about 1003. It contains over 99 per cent, of water. Among the solid 

 constituents sodium chloride is the most prominent it may contain from 

 0-3 to 0-5 per cent, of this salt. It is generally hypotonic as compared with 

 the blood-plasma. It may also contain small traces of protein. This 

 constituent is especially marked in the horse. It generally contains also a 

 small quantity of urea, which may become a prominent constituent in cases 

 of renal disease. The quantity of sweat excreted in the day is very variable. 

 The secretion is under the control of the central nervous system and is 

 almost entirely adapted to the regulation of the body temperature. The 

 nervous mechanism can be set into activity either centrally or reflexly. 

 The most usual factor is a rise of the body temperature. If a man sit in a 

 warm room, e.g. of a Turkish bath, the secretion of sweat commences as soon 

 as the temperature of the body has attained a height of 0-5 to 1 C. above 

 normal. In the case of muscular exercise the temperature will generally be 

 found to be raised if it be taken at the instant that sweating has commenced. 

 The effect of rise of temperature may, however, be either local or central, 

 so that one arm enclosed in a hot-air bath may sweat while the rest of the 

 body is dry. Under ordinary circumstances the central stimulation by the 

 warm blood is the predominant factor. This is shown by an experiment 

 of Luchsinger. In the cat sweating is to be observed only on the hairless 

 pads of the front and hind paws. If one sciatic nerve be cut and the animal 

 be placed in a warm chamber, sweating will commence as the temperature of 

 the animal rises in the three intact paws, while the paw with the nerve cut 

 will be quite dry. Sweating moreover, as has been shown by Kahn, may 

 be induced in the cat's paws by warming the blood passing through the 



