THE SKIN AND THE SKIN GLANDS 1217 



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 

 carotid arteries on its way to the brain, at a time when the temperature of 

 the blood circulating through the rest of the body, including the paws them- 

 selves, has undergone no alteration. Sweating may also be aroused by 

 asphyxia, and this result is found even in the spinal cat, i. e. after separation 

 of the spinal centres from the medulla. The secretion of sweat resulting 

 from stimulation of the sweat nerves, although generally associated with 

 increased vascularity of the skin, is not in any way dependent thereon. 

 Thus even in the amputated limb, stimulation of the sciatic nerve may cause 

 the appearance of drops of sweat on the pad of the foot. If the sciatic nerve 

 be stimulated in the intact animal, the secretion of sweat which is produced 

 is associated with constriction of the vessels of the skin, due to simul- 

 taneous stimulation of the vaso -constrictor nerves running in the sciatic 

 nerve. 



As Langley has shown, the sweat nerves run entirely in the sympathetic 

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