346 SECRETION. 



permanently destroyed by depriving the part of blood for a long time. These facts 

 are very striking and they show a certain similarity between glandular and muscular 

 irritability, although their properties are manifested in very different ways. 



Mechanism of the Production of the Excretions. Certain of the glands have the func- 

 tion of separating from the blood excreinentitious 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 production from the 

 characteristic elements of the secretions. The formation of excrementitious principles 

 takes place in the tissues and is connected with the general process of nutrition ; and in 

 the excreting glands there is simply a separation of matters already formed. The action 

 of the excreting organs being constant, there is not that regular, periodic increase in the 

 activity of the circulation which is observed in secreting organs ; but it has been observed 

 that the blood which comes from the kidneys is nearly as red as arterial blood, showing 

 that the quantity of blood which this organ receives is greater than is required for mere 

 nutrition, the excess, as in the secreting organs, furnishing the water and inorganic salts 

 that are found in the urine. It has also been shown that, when the secretion of urine is 

 interrupted, the blood of the renal veins becomes dark, like the blood in the general 

 venous system. 



The function of excretion is not, under all conditions, confined to the ordinary excre- 

 tory organs. When their action is disturbed, certain of the secreting glands, as the 

 follicles of the stomach and intestine, may for a time eliminate excrementitious matters ; 

 but this is abnormal and is 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 nor- 

 mal conditions, the composition of the blood has little to do with the action of the secret- 

 ing 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 glandular 

 organs, both secretory and excretory. These organs seem to possess a power of selection 

 in the elimination of different substances. Thus, sugar, ferrocyanide of potassium, 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 an 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 secretion. The experiments of Bernard on this point 

 show the influence of pressure upon 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 diminished by the abstrac- 

 tion of blood, which was followed by a corresponding diminution in the quantity of urine. 

 The same phenomena were observed in analogous experiments upon the submaxillary 

 secretion. These striking facts, as we have already seen, do not demonstrate that secretion 

 is due simply to an increase in the pressure of blood in the glands, although this undoubt- 

 edly exerts an important influence. It is necessary that every condition should be 

 favorable to the act of secretion for this influence to be effective. Experiments have 

 shown that pain is capable of completely arresting the secretion of urine, operating 

 undoubtedly through the nervous system. If the flow of urine be arrested by pain, 

 an increase in the pressure of blood in the part fails to influence the secretion. To illus- 



