ABSORPTION OF FATS AND ESTSOLTJBLE SUBSTANCES. 459 



It is probable that some of the watery portions of the 

 urine are reabsorbed by the mucous membrane of the urinary 

 bladder, when the urine has been long confined in its cavity ; 

 though this resorption is ordinarily very slight. A great 

 many cases of discharge of urinary matters by the stomach 

 and intestines, skin, etc., when the urine has been long re- 

 tained, have been reported by the older physiologists, and 

 were supposed to indicate resorption of these principles from 

 the bladder. The mechanism of the excretion of urinary 

 matters was not understood before the experiment^ of Pro- 

 vost and Dumas, who showed that urea will accumulate in 

 the blood after the extirpation of both kidneys in the inferior 

 animals. 1 It is now generally admitted that this takes place 

 when the function of excretion of urine is seriously inter- 

 fered with, and that an attempt is made by Nature to re- 

 move these effete principles from the system by the stomach, 

 intestine, skin, and lungs. It is possible, therefore, that the 

 vicarious discharge of urinary matters in the cases reported, 

 before the true process of excretion by the kidneys was un- 

 derstood, was due to accumulation of the constituents of the 

 urine in the blood, and not their resorption from the urinary 

 passages. 



Absorption may take place from the ducts and the par- 

 enchyma of glands, though this occurs chiefly when foreign 

 substances have been injected into these parts. 



Absorption of Fats and Insoluble Substances. 



The general proposition that all substances capable of 

 being absorbed are soluble in water or in the digestive fluids 

 must be modified in the case of the fats. These are never 

 dissolved in any appreciable quantity in digestion, the only 

 change which they undergo being a minute subdivision in 

 the form of a very fine emulsion. In this condition, the fats 



1 PROVOST ET DUMAS, Examen du Sang et de son Action dans les divers 

 PMnomenes de la Vie. Annales de Chimie et de Physique, Paris, 1823, tome 

 xxiii., p. 90. 



