MECHANISM OF PRODUCTION OF TRUE SECRETIONS 2/1 



regard to the application of this principle to the glands generally, both 

 secretory and excretory. Indeed, it is well known to pathologists, that 

 when the secreting tubes of the kidney have become denuded of their 

 epithelium, they are no longer capable of separating from the blood the 

 peculiar constituents of the urine. 



As regards the origin of the characteristic constituents to the true 

 secretions, it is impossible to entertain any other view than that they 

 are produced in the epithelial structures of the glands. While the 

 secretions contain inorganic salts in solution derived from the blood, the 

 organic constituents, such as ptyalin, pepsin and trypsin, are readily 

 distinguished from all other albuminous substances by their peculiar 

 physiological properties. 



It may be stated, then, as a general proposition, that the character- 

 istic constituents of the true secretions, as contradistinguished from the 

 excretions, are formed by the epithelial structures of the glands, out of 

 materials furnished mainly by the blood. Their formation is not con- 

 fined to what is usually termed the period of activity of the glands, or 

 the time when the secretions are poured out, but it takes place more or 

 less constantly when no liquid is discharged. It is more than probable, 

 indeed, that the formation of the peculiar and characteristic constituents 

 of the secretions takes place with as much, or even more activity in the 

 intervals of secretion as during the discharge of liquid; and most of 

 the glands connected with the digestive system seem to require certain 

 intervals of repose and are capable of discharging their secretions for a 

 limited time only. This condition of a gland is called resting. 



When a secreting organ is called into activity like the gastric 

 mucous membrane or the pancreas, following the introduction of food 

 into the alimentary canal a marked change in its condition occurs. 

 The circulation in the part is then much increased in activity, thus fur- 

 nishing water and the inorganic constituents of the secretion. This 

 difference in the quantity of blood in the glands during their activity 

 may be observed when the organs are exposed in a living animal and is 

 one of the important conditions bearing on the mechanism of secretion. 

 In all the secretions proper, then, there are intervals, either of complete 

 repose, as is the case with the gastric juice or the pancreatic juice, or 

 periods when the activity of the secretion is greatly diminished, as in 

 the case of the saliva. The resting periods are necessary to the proper 

 action of the secreting glands ; forming a marked contrast with the con- 

 stant action of organs of excretion. It is well known, for example, that 

 digestion is seriously disturbed when the act is too prolonged on account 

 of the habitual ingestion of an excessive quantity of food. 



While the mechanism of secretion is not understood in all its details 



