Chap, xviii.] SECRETIONS AND EXCRETIONS IN PARTICULAR. 271 



profound of the organic substances, is formed by disassimilation 

 in the anatomical elements themselves. Possibly, however, the 

 uric acid, the result of an incomplete oxydation, is produced for 

 the most part in the blood itself, when the fluid is surcharged 

 with peptones which the tissues are not able to assimilate. 



But how can we explain the fact that uric acid is found in 

 such enormous proportion precisely in those of the vertebrates 

 which are endowed with an extreme respiratory activity, namely, 

 in birds, which, however, as regards uric secretion, resemble 

 testaceous reptiles ] Yerily in both the uric acid is found in so 

 great a quantity, that it is concreted and crystallised even in the 

 interior of the urinary canals, and gives to the urine the appear- 

 ance of a whitish pulp. Among the invertebrates^ concretions 

 and crystals of like nature are not met with even in the cells of 

 the urinary glands. 



We can view as having affinity to the urinary excretion the 

 sudoral excretion, operated by millions of cutaneous glandules, 

 each of which may be considered as a small kidney excreting an 

 aqueous solution, charged with substances analogous to those 

 which are found in the urine. But the sudoral excretion is 

 much less rich in salts and in organic matters than the urine. 

 "We usually find in it urea, but no uric acid or urates. 



Secretion, we have said, can be traced back to phenomena of 

 nutrition, that is to say, to molecular acts, effected in the midst 

 of glandular cells, which means that it can be accomplished 

 without the intervention of the nervous system. Such is evi- 

 dently the case with vegetal secretions. But in the superior 

 animals, having a complete nervous system and suitable capillary 

 vessels, the secretion evidently depends on the degree of reple- 

 tion of these vessels, and on the rapidity with which the 

 sanguineous current traverses them. It is therefore indispen- 

 sable that the secretion should be influenced by the vaso-motory 

 nerves ; and this is in effect what experiment demonstrates. 

 Normally, secretion is always accompanied by a dilatation of the 

 capillaries of the gland, by a congestion, and every congestion 



