356 LIFE : OUTLINES OF GENERAL BIOLOGY 



the cortex of the kidney, they end in tiny filters (called glomeruli). 

 The theory of the action of these structures is still disputable, but 

 Cushny's account of it is the most complete and satisfactory. 

 According to this view, the glomeruli are simple filters of the type 

 suggested above, but the tubules in the kidney medulla reabsorb 

 water, salts, and useful dissolved substances from the filtrate, and 

 retain them witliin the body, while they refuse to absorb waste- 

 products. The filtrate passing the glomeruli, then, has the composi- 

 tion of the blood minus its colloids — that is, much water, various 

 salts, some food materials (glucose), and a relatively small amount 

 of various waste-products. The tubules tend to absorb from this 

 filtrate a fluid having the composition of the useful part of normal 

 blood, containing no waste-products. All the waste-products which 

 pass the glomerulus therefore pass the tubules also and are dis- 

 charged. But if any of the useful constituents of blood be present in 

 too great amount, whether water, or salts, or glucose, then the 

 excess will not be absorbed by the tubules (which take out only the 

 quantity contained in normal blood), but will be discharged. So that 

 when water or salts or glucose pass a certain limit in the blood, 

 and therefore in the glomerular filtrate, the excess is got rid of from 

 the body; they are therefore spoken of as "threshold" substances, 

 whereas the waste products like urea are excreted however little 

 may be present, and are called "No-threshold" bodies. 



The total action of the kidney, then, is to remove from the blood 

 all the waste-products, and the excess of the "threshold" substances 

 such as water, salts, and glucose (the latter only attaining the thres- 

 hold in experimental conditions, or in the disease of diabetes). It 

 should be noticed that the urine is a more concentrated fluid than 

 the blood, and therefore the kidney performs actual work and 

 expends energy, since work must be done to concentrate a fluid. 

 The kidney, of course, is not a perfect organ; it allows traces of 

 useful substances to escape; especially in disordered conditions, to 

 which, unfortunately, it is only too subject. 



Animal Heat. — Animals obviously expend energy in moving; 

 birds and mammals usually have to expend energy to maintain 

 their body temperature at its constant level; all living cells expend 

 energy in many subtler ways. The chief subjects of physiological 

 study are the sources of this energy; the means by which it is made 

 available; the ways in which it is expended, and the control of this 

 expenditure. The analogy between an animal and a candle, which 

 also expends energy and which also "uses up" air and "dies" in a 

 confined space, was thoroughly appreciated in the early eighteenth 

 century; but it was not known what the candle did to the air in 

 which it burned, still less how animals transformed the air they 

 breathed. 



