SECRETIONS 103 



the glands of Brunner in the duodenum and of the crypts of 

 Lieberkiihn in the whole intestine. Concerning the secre- 

 tion of Brunner's glands and the conditions of their secre- 

 tion nothing is known. 



The crypts of Lieberkiihn of the small intestine are 

 simple tubular glands, placed in thick clusters between the 

 villi of the mucous membrane. They secrete the intestinal 

 juice containing the diastatic ferment. The secretion takes 

 place when the mucous membrane of the intestine is directly 

 or reflexly stimulated by the taking up of food. As the 

 secretion also takes place in those parts of the intestine not 

 directly stimulated, it is dependent upon the nervous system. 

 The secretory nerves are, however, not known. 



The muciparous glands of the large intestine contain many 

 globlet cells forming mucus; these globlet cells are only 

 found occasionally in the glands of the small intestine. 



The intestinal glands also seem to regenerate the epithelial 

 glands of the villi. In the intestinal glands new cells are con- 

 tinually formed by mitotic division; these new cells pass upward 

 to take the place of the broken-down epithelial cells of the free 

 surface of the mucous membrane. 



7. RENAL SECRETION 



i. Composition of urine. Urine, the secretion of the 

 kidneys, is, in man, a yellow or reddish-brown fluid having 

 a specific gravity of 1.017-1.040. Its reaction is generally 

 acid (due to acid sodium phosphate), but after a meal of 

 vegetables containing the salts of vegetable acids, which in 

 the body form carbonates, it may be neutral or alkaline. 

 Its reaction is alkaline also during the period of greatest 

 gastric digestion because the alkalinity of the blood is 

 increased on account of the acid formation in the stomach, 

 and this excess of alkali is excreted in the urine. 



When urine stands for a certain length of time, putrefaction sets 

 in by which the urea is changed to ammonium carbonate (alkaline 

 urea fermentation) ; this causes the urine to become alkaline. 

 Such alkaline urine is cloudy because of a precipitate of ammonio- 



