18 SECRETION. 



Mechanism of the Production of the true Secretions. 

 Although the characteristic elements of the true secretions 

 are not to be found in the blood or in any other of the 

 animal fluids, they can generally be extracted in quantity 

 from the glands, particularly during their intervals of repose. 

 This fact has been repeatedly demonstrated with regard to 

 many of the digestive fluids, as the saliva, the gastric juice, 

 and the pancreatic juice ; and artificial fluids, possessing 

 many of the physiological properties of the natural secre- 

 tions, have been prepared by simply infusing the glandular 

 tissue in water. There can be no doubt, therefore, that even 

 during the periods when the secretions are not discharged, 

 the glands are taking from the blood matters which are to 

 be transformed into principles characteristic of the individual 

 secretions, and that this process is constant. Extending our 

 inquiries into the nature of the process by which these pecu- 

 liar principles are formed, it is found to bear a close resem- 

 blance to the general act of nutrition. There are certain 

 anatomical elements in the glands which have the power 

 of selecting the proper material from the blood and causing 

 them to undergo a catalytic transformation ; as the muscu- 

 lar tissue takes from the great nutritive fluid the albumen, 

 fibrin, etc., and transforms them into its own substance. The 

 exact nature of this property is unexplained ; it belongs to 

 the class of phenomena observed in living structures only, 

 and is sometimes called vital. 



In all of the secreting organs a variety of epithelium is 

 found, called glandular, which seems to possess the power 

 of forming the peculiar elements of the different secretions. 

 Inasmuch as the epithelial cells lining the tubes or follicles 

 of the glands constitute the only peculiar structures of these 

 parts, the rest being made up of basement-membrane, con- 

 nective tissue, blood-vessels, nerves, and other structures 

 which are distributed generally in the economy, we should 

 expect that these alone would contain the elements of the 

 secretions. In all probability this is the fact ; and with re- 



