PRODUCTION OF THE EXCRETIONS. 311 



secretion may be excited without any modification of the circulation. This 

 excitability will disappear when the artery supplying the part with blood is 

 tied for a number of hours ; and secretion can not then be excited even 

 when the blood is again allowed to circulate. If the gland be not deprived 

 of blood for too long a period, the excitability is soon restored ; but it may 

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

 These facts show a certain similarity between glandular and muscular excita- 

 bility, although these properties are manifested in very different ways. 



Mechanism of the Production of the Excretions. Certain of the glands 

 separate from the blood excrementitious matters which are of no use in the 

 economy and are simply discharged from the body. 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 constituents of the secretions. 

 The formation of excrementitious matters 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 products already formed. The action 

 of the excreting organs is constant, and 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 kid- 

 neys is nearly as red as arterial blood, showing that the quantity of blood 

 which these organs receive 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 secre- 

 tion of urine is interrupted, the blood of the renal veins becomes dark like 

 the blood in the general venous system. 



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 excre- 

 mentitious 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 normal conditions, the composition of the blood has little to do with 

 the action of the secreting organs, as it simply furnishes the materials out of 

 which the characteristic constituents of the secretions 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 and potassium fer- 

 rocyanide 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 discharge of secretions is almost always accompanied with an in- 

 crease 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 



