EXCRETION 473 



is no longer emptied ; it remains distended with urine, which 

 dribbles away through the urethra as fast as it escapes from 

 the ureters. To this condition the term incontinence of urine is 

 properly applied. 



Reflex emptying of the bladder, without an act of will or 

 during unconsciousness, is not true incontinence. The in- 

 voluntary micturition of children during sleep, for example, 

 is a perfectly normal reflex act, although more- easily excited 

 and less easily controlled than in adults. Section either of both 

 nervi erigentes, or of both hypogastrics, is never followed by 

 more than quite temporary disturbance of function of the bladder 

 in dogs, both male and female. In a few days the urine is 

 normally passed. In bitches the same is true when both pairs 

 of nerves are divided. But in male dogs true incontinence of 

 urine follows section of the four nerves, as well as intense tenesmus 

 due to paralysis of the lower part of the large intestine. 



II. Excretion by the Skin. 



Besides permitting of the trifling gaseous interchange already 

 referred to (p. 275), the skin plays an important part in the 

 elimination of water by the sweat-glands. 



Sweat is a clear colourless liquid of low specific gravity (1003 

 to 1006), consisting chiefly of water with small quantities of 

 salts, neutral fats, volatile fatty acids, and the merest traces 

 of proteins and urea. It is acid to litmus except in profuse 

 sweating, when it may become neutral or even alkaline. It is 

 secreted by simple gland-tubes, which form coils lined with a 

 single layer of columnar epithelium, in the subcutaneous tissue, 

 with long ducts running up to the surface through the true skin 

 and epidermis. Unless collected from the parts of the skin on 

 which there are no hairs, such as the palm, it is apt to be mixed 

 with sebum, a secretion formed by the breaking down of the 

 cells of the sebaceous glands, which open into the hair follicles, 

 and consisting chiefly of glycerin and cholesterin fats, soaps, and 

 salts. Sebum is probably of considerable importance for main- 

 taining the normal condition of the hair and skin. 



Although it is only occasionally that sweat collects in visible 

 amount on the skin, water is always being given off in the form 

 of vapour. This invisible perspiration leaves behind it on the 

 skin, or in the glands, the whole of the non-volatile constituents, 

 which may be to some extent reabsorbed ; and since even the 

 visible perspiration is in large part evapoiated from the very 

 mouths of the glands in which it is formed, the sweat can hardly 

 be considered a vehicle of solid excretion, even to the small 

 extent indicated by its chemical composition. 



The total quantity of water excreted by the skin, and the 



