PRODUCTION OF THE TRUE SECRETIONS. 343 



filtration from the blood. They always preexist in the circulating fluid and may be elim- 

 inated, either constantly or occasionally, by a number of organs. As they are produced 

 continually in the substance of the. tissues and are taken up by the blood, they are con- 

 stantly discharged into the substance of the proper eliminating organs. "When the glands 

 which thus eliminate these principles are destroyed or when their functions are serious- 

 ly impaired, the excrementitious matters may accumulate in the blood and give rise to 

 certain toxic phenomena. These effects, however, are often retarded by the vicarious 

 discharge of such principles by other organs. 



There are some fluids, as the bile, which perform important functions as secretions, 

 and which nevertheless contain certain excrementitious matters. In these instances, it is 

 only the excrementitious matters that are discharged from the organism. 



In the sheaths of some tendons and of muscles, the substance of muscles, and in 

 some other situations, are found fluids which simply moisten the parts, and which contain 

 very little organic matter, with but a small proportion of inorganic salts. Although 

 these are frequently spoken of as secretions, they are produced generally by a simple, 

 mechanical transudation of certain of the constituents of the blood through the walls 

 of the vessels. Still, it is difficult to draw a line rigorously between transudation and 

 some of the phenomena of secretion; particularly as late experiments upon dialysis have 

 shown that simple, osmotic membranes are capable of separating complex solutions, allow- 

 ing certain constituents to pass much more freely than others. This fact explains why 

 the transuded fluids do not contain all the soluble principles of the blood in the propor- 

 tions in which they exist in the plasma. All the secreted fluids, both the true secretions 

 and the excretions, contain many of the inorganic salts of the blood-plasma. 



Mechanism of the Production of the True Secretions. Although the characteristic ele- 

 ments 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 secretions, 

 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 peculiar principles are formed, 

 it is found to bear a close resemblance 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 peculiar transformation ; in the same way 

 that the muscular tissue takes from the great nutritive fluid albuminoid matters and 

 transforms them into its own substance. The exact nature of this property is unex- 

 plained. 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, 

 connective tissue, blood-vessels, nerves, and other structures which are distributed gen- 

 erally 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 regard to some of the glands, 

 this has been satisfactorily demonstrated. It has been found, for example, that the liver- 

 cells contain the glycogenic matter formed by the liver ; and it has been farther shown 

 that, when the cellular structures of the pancreas have been destroyed, the secretion is 

 no longer produced. There can be hardly any doubt with regard to the application of 



