CHAPTER II 



SECRETION 



THE term secretion may be used to designate either the 

 liquid or semiliquid products of glandular organs which are 

 discharged upon free or closed surfaces; or to designate the 

 process itself by which these products are formed. According 

 as the surface is free (skin, mucous membrane) or closed (blood 

 and lymph cavities) the secretion is termed an external or an 

 internal secretion. Such substances serving a useful purpose 

 are typical secretions; when of no further use, are excretions. 

 There is no longer any doubt that gland cells are active in the 

 formation of their secretions. The proofs are : 



1. The gland cells undergo a microscopic change. 



2. Specific substances in the secretion which are not found 

 in the blood or lymph. 



3. The liberation of energy in the form of heat, pressure, 

 and electricity. 



4. The results of the stimulation of the nerve supply. 



5. The action of certain drugs. 



The processes of filtration, diffusion, and osmosis cannot, 

 however, be entirely excluded from acting in conjunction with 

 the physiological properties of the living structure of gland 

 cells. By filtration is meant the passage of fluids through a 

 membrane as the result of differences of hydrostatic pressure. 

 Diffusion is the interpenetration of the molecules of two fluids 

 when brought into contact. Osmosis is the diffusion of water 

 that takes place through membranes separating two solutions. 



Salivary Glands. The production of saliva is brought 

 about by the joint action of three larger pairs of glands, the 

 parotids, submaxillaries, and sublinguals, and by innumerable 

 smaller ones lying in the mucous membrane of the mouth and 

 tongue. 



