MODIFICATIONS OF SECRETION. 27 



analogous to the elimination of foreign matters from the blood 

 by the glands. 



Influence of the Composition and Pressure of the Blood 

 upon Secretion. Under normal conditions the composition 

 of the blood has little to do with the action of the secreting 

 organs, as it simply furnishes the material out of which the 

 characteristic principles of the secretion are formed ; ' but 

 when certain foreign matters are taken into the system or 

 are injected into the blood-vessels, they are eliminated by the 

 different glands, both secretory and excretory. These organs 

 seem to possess a power of selection in the elimination of 

 different substances. Thus, sugar, ferrocyanide of potas- 

 sium, and the salts of iron, are eliminated in greatest quantity 

 by the kidneys ; the salts of iron by the kidneys and the 

 gastric tubules ; and iodine by the salivary glands. 



The act of secretion is almost always accompanied with 

 increase in the pressure of blood in the vessels supplying the 

 glands ; and it has been shown, on the other hand, that an 

 exaggeration in the pressure, if the nerves of the glands do 

 not exert an opposing influence, increases the activity of se- 

 cretion. The experiments of Bernard on this point show the 

 influence of pressure on the salivary and the renal secretion, 

 particularly the latter. After inserting a tube into one of 

 the ureters of a living animal, so that the activity of the 

 renal secretion could be accurately observed, the pressure in 

 the renal artery was increased by tying the crural and the 

 brachial. It was then found that the flow of urine was 

 markedly increased. The pressure was afterward dimin- 

 ished by the abstraction of blood, which was followed by a 

 corresponding diminution in the quantity of urine. 1 The 

 same phenomena were observed in analogous experiments on 

 the submaxillary secretion. 



These striking facts, as we have already seen, do not de- 

 monstrate that secretion is due simply to an increase in the 



1 BERNARD, Liquids de rorganisme, Paris, 1859, tome ii., p. 155, et seq. 



