INFLUENCE OF THE BLOOD ON SECRETION 273 



The theory that the discharge of the secretions is due simply to 

 mechanical causes and is attributable solely to the increase in the press- 

 ure of blood can not be sustained. Blood-pressure undoubtedly has 

 considerable influence on the activity of secretion ; but the flow will not 

 always take place in obedience to simple pressure, and secretion may be 

 excited for a limited time without any increase in the quantity of blood 

 circulating in the gland. 



The glands possess a peculiar excitability, manifested by their action 

 in response to proper stimulation. During secretion they usually receive 

 an increased quantity of blood ; but this is not indispensable, and secre- 

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

 excitability disappears after the artery supplying the part with blood 

 has been 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 

 is not deprived of blood for too long a period, however, the excitability 

 is soon restored. 



Mechanism of the Production of the Excretions. Certain glands 

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

 the economy and are discharged from the body. These matters are 

 different in their mode of production from the Characteristic constituents 

 of the secretions. Their formation takes place in the tissues and is con- 

 nected with the general process of nutrition ; and in the excreting glands 

 there is simply a separation of products already formed, probably by 

 cellular action similar to that observed in secreting organs. The action 

 of the excreting organs is constant, and there is not that regular periodic 

 increase in the activity of the circulation observed in secreting organs ; 

 but it has been noted that the blood coming from the kidneys is nearly 

 as red as arterial blood, showing that the quantity of blood which these 

 organs receive is greater than the amount 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. 



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 analo- 

 gous to the elimination of foreign matters from the blood by the glands. 



Influence of the Composition and Pressure of the Blood on Secretion. 

 Under normal conditions, the composition of the blood has little to do 

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

 out of which the characteristic constituents of the secretions are formed ; 



