MECHANISM OF EXCKETIOX. ZO 



lated and the blood is again allowed to circulate. If the gland 

 be not deprived of blood too long, the irritability is soon re- 

 stored ; but it may be permanently destroyed by depriving the 

 part of blood for a long time. 1 These observations are very 

 striking, and show a certain similarity between glandular 

 and muscular irritability, though their properties are mani- 

 fested in very different ways. 



Mechanism of the Production of the Excretions. Certain 

 of the glands have the function of separating from, the blood 

 excrementitious matters, which are of no use in the economy, 

 and are simply to be discharged from the system. These 

 matters, which will be fully considered, both in connection 

 with the fluids of which they form a part, and under the 

 head of nutrition, are entirely different in their mode of pro- 

 duction from the characteristic elements of the secretions. 

 Our definite information concerning the mechanism of ex- 

 cretion dates from the researches of Prevost and Dumas, who 

 discovered urea in the blood of dogs after its elimination had 

 been arrested by extirpation of the kidneys. 2 These experi- 

 ments were confirmed by Segalas and Yauquelin ; 3 but at 

 that time the means of analysis of the animal fluids were not 

 sufficiently delicate to enable chemists to detect urea in 

 healthy blood. The later observations of Marchand, how- 

 ever, demonstrated its constant presence in very small 

 quantity in the blood.* These analyses have been repeated- 

 ly confirmed, and it is now generally believed that all the 

 excrementitious principles exist in greater or less quantity 



1 Unpublished lectures delivered by Bernard at the College of France in the 

 summer of 1861. 



2 PREVOST ET DUMAS, Examin du sang et de son action dans les divers plie- 

 nomenes de la vie.Annales de ckimie et de physique, Paris, 1821, tome xviii., p. 

 280. 



3 SEGALAS, Sur des nouvelles experiences relatives aux propriete* medicamen- 

 teuscs de Turce, etc. Journal de physiologic, Paris, 1822, tome ii., p. 354. 



4 MARCHAND, Sur la presence de Turee dans le sang. Annales des sciences 

 naturelles, Paris, 1838, 2me serie, tome x., p. 46. 



