174 ANIMAL PHYSIOLOGY. 



definite arrangement; for they can be shown to consist of 

 multitudes of figures of eight, with the crossings of the figures 

 in front and behind, and their loops round the summit and 

 outlet (Pettigrew). The fibres are of the unstriped descrip- 

 tion ; but round the urethra or canal of exit, where it emerges 

 from the pelvis, there are muscles of the striped kind, by 

 whose action the contents of the bladder are voluntarily 

 retained, when its walls are irritated to contract by the 

 stimulus of distension. 



132. The Urine. The average amount secreted by the 

 kidneys is about fifty fluid ounces, or two pints and a half, 

 in the day; the reaction is usually acid, and the average 

 specific gravity is 1*020; but the amount of water secreted 

 by this channel is affected not only by the amount imbibed, 

 but by various circumstances, particularly temperature and 

 exercise, which influence the quantity carried away by the 

 skin and lungs. Whatever diminishes the amount of water 

 secreted by the kidneys, or increases the quantity of waste 

 substance, heightens the specific gravity of the urine. 



The solid contents of the urine consist of nitrogenous 

 matters and salts. The salts are chlorides, sulphates, and 

 phosphates. Chlorides occur in all the fluids of the body; 

 the sulphates take origin, no doubt, by the oxidation of the 

 sulphur contained in albuminoid matters; and the phosphates 

 are derived from the oxidation of the phosphorus in albumi- 

 noids, and in the protagon of the nervous system, and from 

 the phosphates pre-existing in the bones and various fluids. 



The principal nitrogenous matter in the urine is urea, 

 which, as already stated, is the substance in the form of 

 which by far the greater part of the nitrogen escapes from 

 the body. It is exceedingly soluble, and contains exactly 

 one half its weight of nitrogen. About 500 grains of urea 

 may be said to escape by the kidneys in twenty-four hours. 

 There is contained in the urine of man a small quantity oi 

 hippuric acid, a substance which gets its name from being 

 found abundantly in the urine of the horse; and there is 

 likewise ordinarily a small quantity of uric acid, which is 

 important; because, from slight derangements of nutrition, 

 its amount is liable to increase, and when it accumulates 

 in the blood it produces gout, while, in the urine, on 



