50 COMPARATIVE PHYSIOLOGY 



of the kidneys involves oxidation of carbohydrate material, 

 as in the case of striped muscle. 



It has been seen in a previous chapter that activity of muscle 

 and cilia depends on an anaerobic reaction, oxidative processes 

 being associated only with the recovery process. That this 

 is true of secretory response is not easily demonstrated in an 

 organ which secretes more or less continuously like the kidney. 

 But in the case of salivary secretion, which can be controlled 

 for the exigencies of experiment by means of its nervous 

 connexions, Barcroft and Piper (19 12) were able to demonstrate 

 that the maximal rate of oxygen consumption occurred 

 appreciably later than maximal rate of flow from the gland ; 

 and that the increased oxygen consumption accompanying 

 secretory activity in the salivary gland of the mammal outlasts 

 by a considerable interval the cessation of active secretion. 

 This clearly points to the probability that in the mechanism 

 of secretion as in that of muscular and ciliary activity oxida- 

 tive reactions are especially characteristic of the recovery 

 phase. 



For a detailed discussion of the evidence concerning the 

 way in which the adjustment of concentration of each of the 

 constituents of the urine is carried out in the renal tubules, the 

 reader should consult Cushny's monograph on the secretion 

 of urine. Two rival hypotheses have been advocated. 

 According to one the renal tubules selectively absorb from 

 the filtrate ; according to the other they differentially remove 

 from the blood appropriate quantities of the urinary consti- 

 tuents. It is certain that dyes are excreted by the cells of the 

 renal tubules, and innumerable researches of this kind have been 

 prosecuted in connexion with the segmental excretory organs 

 and malpighian tubes of Arthropods and the nephridia of 

 worms. But it is difficult to see what light they throw on the 

 normal elimination of salts and nitrogenous waste from the 

 body fluids. The fluid of the excretory glands of molluscs 

 is particularly rich in uric acid and other purine bodies. A 

 filtration mechanism in these animals is, however, excluded 

 by the very low blood-pressures which are found in all Inverte- 

 brates except Cephalopods. It is possible that reabsorption and 



