DISPOSAL OF WASTES 139 



Other glands producing liquid secretions have some chance of casting 

 out soluble wastes. Thus in the saliva there are traces of urea; but since 

 most of the saliva is retained within the body, the occurrence of urea in it 

 hardly amounts to excretion. Drugs injected into the veins can often 

 be tasted owing to a similar exci'etion of them in the salivary glands. The 

 wall of the large intestine is able to excrete small amounts of unusual 

 foreign substances occurring in the blood or of ordinary substances when 

 present in excessive amounts, as calcium and magnesium sometimes are. 

 These substances are removed from the intestine with the feces. 



None of these other excretory organs is important as a substitute for 

 'the kidneys; not even all of them combined could take over the job 

 of the kidneys. Fortunately the kidneys have a wide margin of safety, 

 for a kidney and a half may be removed and the necessary excretion still 

 go on. There is no recovery, however, from overdestruction of kidney 

 tissue, for the renal tubules do not regenerate. 



Some organisms, principally plants, excrete wastes by simply render- 

 ing them insoluble and then retaining them within or between the cells. 

 Insoluble substances can do no harm and, when they are not abundant, 

 are not greatly in the way. Among animals, sea urchins are said to 

 store insoluble excretions. 



References 



Baitsell, G. a. Human Biology. McGraw-Hill Book Company, Inc. (Chap. VI.) 

 Carlson, A. J., and V. Johnson. The Machinery of the Body. University of 

 Chicago Press. 



