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



iprofound of the organic substances, is formed by disassimilation 

 >in the anatomical elements themselves. Possibly, however, the 

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

 ithe 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 

 i testaceous reptiles ? Yerily in both the uric acid is found in so 

 i 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 

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

 I the urinary glands. 



We can view as having affinity to the urinary excretion the 

 isudoral excretion, operated by millions of cutaneous glandules, 

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

 j aqueous solution, charged with substances analogous to those 

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

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

 I "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 

 I of glandular cells, which means that it can be accomplished 

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

 i dently the case with vegetal secretions. But in the superior 

 I animals, having a complete nervous system and suitable capillary 

 I 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- 

 1 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 



