CHAPTER XIV 

 SECRETION 



SECRETION is a term applied to a process by which 

 a portion of the constituents of the blood are separ- 

 ated from the blood-stream, by the activities of the 

 endothelial cells of the capillary walls, as the blood 

 flows through the capillaries. In this process the 

 endothelial cell is aided by the physical forces 

 diffusion, osmosis, and filtration. The materials thus 

 separated are collectively termed lymph (Brubaker). 



These secretions are utilized and adapted to many 

 and complex functions, dependent upon the secre- 

 tory organ which secretes the fluid and the membrane 

 it is poured out upon. They enable the tissues of 

 the body to repair, grow, and produce heat and 

 energy. Others are to promote digestion, etc., remove 

 foreign bodies (dust, etc.) from membranes, as the 

 conjunctiva, to prevent friction between the serous 

 membranes, as the pericardium, pleura, and peri- 

 toneum; and to prevent friction between the ex- 

 tremities of the bones entering into the formation of 

 the joints, as the fluid in synovial membranes. 



Secretions are divided into internal and external 

 secretions. Internal secretions are fluids secreted by 

 the epithelial cells of certain organs of the body which 

 do not possess any ducts by which their secretion 

 is poured into any cavity or organ, but is reabsorbed 

 into the blood, and the contained specific constituents 

 aid in the nutrition of the body. These organs are: 

 the thyroid, thymus, adrenal, spleen, pituitary glands, 

 hypophysis, etc. (See description of the Ductless 

 Glands, page 303.) 



