III. SECRETION. 



A. GENERAL CONSIDERATIONS. 



THE term secretion is meant ordinarily to apply to the liquid or semi- 

 liquid products formed by glandular organs. On careful consideration 

 it becomes evident that the term gland itself is widely applied to a variety 

 of structures differing greatly in their anatomical organization so much so, 

 in fact, that a general definition of the term covering all cases becomes 

 very indefinite, and as a consequence the conception of what is meant by a 

 secretion becomes correspondingly extended. 



Considered from the most general standpoint we might define a gland 

 as a structure composed of one or more gland-cells, epithelial in character, 

 which forms a product, the secretion, which is discharged either upon a 

 free epithelial surface such as the skin or mucous membrane, or upon the 

 closed epithelial surface of the blood- and lymph-cavities. In the former case 

 that is, when the. secretion appears upon a free epithelial surface communi- 

 cating with the exterior, the product forms what is ordinarily known as a 

 secretion ; for the sake of contrast it might be called an external secretion. 

 In the latter case the secretion according to modern nomenclature is designated 

 as an internal secretion. The best-known organs furnishing internal secretions 

 are the liver, the thyroid, and the pancreas. It remains possible, however, 

 that any organ, even those not possessing an epithelial structure, such as 

 the muscles, may give off substances to the blood comparable to the internal 

 secretions a possibility which indicates how indefinite the distinction between 

 the processes of secretion and of general cell-metabolism may become if the 

 analysis is carried sufficiently far. If we consider only the external secretions 

 definition and generalization become much easier, for in these cases the secret- 

 ing surface is always an epithelial structure which, when it possesses a certain 



organization, is designated as 

 a gland. The type upon which 

 these secreting surfaces are con- 

 structed is illustrated in Figure 



FIG. 63. Plan of a secreting membrane. _.. 



63. The type consists of an 



epithelium placed upon a basement membrane, while upon the other side of 

 the membrane are blood-capillaries and lymph-spaces. The secretion is 

 derived ultimately from the blood and is discharged upon the free epithelial 

 surface, which is supposed to communicate with the exterior. The mucous 

 membrane of the alimentary canal from stomach to rectum may be considered, 



152 



