PART VII 

 SECRETION 



SECTION XXIV 

 THE ** EXTERNAL'' SECRETIONS 



CHAPTER LXXVI 



THE GROUP OF THE CUTANEOUS SECRETIONS 



Classification of the Secretions. — A secretion is a cellular product 

 which is of further use to the body, while an excretion is a cellular 

 product which is of no further use to the body. Obviously, this 

 definition must be held within very general limits, because not all 

 secretions and excretions are fiuids. It will also be remembered that 

 the secretions are formed by special colonies of cells which Dresent 

 themselves as ''externar' and 'in- 

 ternal" glands. The former possess 

 a visible duct through which the 

 secretion escapes, while the latter 

 do not, and constitute, therefore, the 

 group of the so-called ductless glands. 

 Consequently, while the '' external" 

 secretions are poured upon an open 

 surface of the body, the internal se- ' ^/ 



cretions are discharged directly into ^'^' 



the blood or lymph stream. For this Fi«- 486.— Diagrammatic Representa- 

 ,^ (. , n TiON OF AN Acinus. 



reason, the former most generally r» t^ ^ c ^ n r i v, 



,' . ^ -^ D, Duct; S, secretory cells; L, lymph 



give rise to local reactions, while the space; C, blood capillaries. 



latter are distributed throughout the 



body and aid in the promulgation of physiological processes of a more 

 general and intricate kind. This relatively sharp line of demarcation 

 already drawn between the '^external" and '^ internal" secretions, may 

 be elucidated further by briefly noting the histological character of 

 the glands producing them. The external glands invariably exhibit 

 a structure which betrays its secretory nature almost immediately, 

 while that of the internal glands is generally obscure and cannot 

 readily be associated with secretion. 



A secreting mechanism consists essentially of a colony of cells 

 which are arranged around a central cavity or tube for the reception 



891 



