154 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



glands, the ovary and testis, whose right to the designation of glands is doubt- 

 ful, we may say that the secretions in the mammalian body are liquid or semi- 

 liquid in character and are composed of water, inorganic salts, and various 

 organic compounds. With regard to the last-mentioned constituent the secre- 

 tions differ greatly. In some cases the organic substances present are not found in 

 the blood, and furthermore they may be specific to a particular secretion, so that 

 we must suppose that these constituents at least are constructed in the gland 

 itself. In other cases the organic elements may be present in the blood, and 

 are merely eliminated from it by the gland, as in the case of the urea found in 

 the urine. Johannes Muller long ago made this distinction, and spoke of secre- 

 tions of the latter kind as excretions, a term which we still use and which car- 

 ries to our minds also the implication that the substances so named are waste 

 products whose retention would be injurious to the economy. Excretion as 

 above defined is not a term, however, which is capable of exact application to 

 any secretion as a whole. Urine, for example, contains some constituents which 

 are probably formed within the kidney itself, e. g. hippuric acid ; while, on 

 the other hand, in most secretions the water and inorganic salts are derived 

 directly from the blood or lymph. So, too, some secretions for example, the 

 bile carry off waste products which may be regarded as mere excretions, and 

 at the same time contain constituents (the bile salts) which are of immediate 

 value to the whole organism. Excretion is therefore a name which we may 

 apply conveniently to the process of removal of waste products from the body, 

 or to particular constituents of certain secretions, but no fundamental distinc- 

 tion can be made between the method of their elimination and that of the 

 formation of secreted products in general. Owing to the diversity in com- 

 position of the various external secretions and the obvious difference in the 

 extent to which the glandular epithelium participates in the process in different 

 glands, a general theory of secretion cannot be formulated. The kinds of 

 activity seem to be as varied as is the metabolism of the tissues in general. 



It was formerly believed that the formation of the secretions was de- 

 pendent mainly if not entirely upon the physical processes of filtration, im- 

 bibition, and diffusion. The basement membrane with its lining epithelium 

 was supposed to constitute a membrane through which various products of the 

 blood or lymph passed by filtration and diffusion, and the variation in com- 

 position of the secretions was referred to differences in structure and chemical 

 properties of the dialyzing membrane. The significant point about this view 

 is that the epithelial cells were supposed to play a passive part in the process ; 

 the metabolic processes within the cytoplasm of the cells were not believed to 

 affect the composition of the secreted product. As compared with this view 

 the striking peculiarity of modern ideas of secretion is, perhaps, the import- 

 ance attributed to the living structure and properties of the epithelial cells. 

 It is believed generally now that the glandular epithelium takes a direct part 

 in the production of some if not all of the constituents of the secretions. The 

 reasons for this view will be brought out in detail further on in describing the 

 secreting processes of the separate glands. Some of the general facts, how- 



