4 PHYSIOLOGY CHAP. 



(e) The special characteristics and differences in the secretions 

 depend not on any external and mechanical change, nor upon 

 the anatomical form of the gland, but solely upon the specific 

 character of the living organic substance (epithelium) which 

 invests the internal secreting ducts. The difference in secretions 

 depends, therefore, upon the same cause which determines differ- 

 ences of conformation and of life of the organs in general : there 

 is but one difference, i.e. in the one case the altered blood is 

 incorporated with the organ, in the other it passes beyond its 

 limits, and appears externally to it, in the form of secretion. 



(/) The chemical processes carried out in the secretory organs 

 are twofold. On the one hand, they serve the nutrition, develop- 

 ment, or formation of new cells ; on the other, the formation of a 

 heterologous product of secretion. The secreting cells differ 

 chemically from the product secreted, although they may contain 

 a small amount of the latter. Secretion cannot, therefore, be 

 explained as a simple liquefaction of the pre-existing molecules of 

 the secretory elements. We must assume that the products of 

 secretion are gradually perfected in what may be a long journey 

 through the canaliculi of the gland. 



This conception of the general morphology and physiology, of 

 the secreting glands as formulated by Johannes Mu'ller still holds 

 good, and is a fitting introduction to the special study of the 

 functions of the individual organs and mechanisms of secretion. 

 Subsequent work has supplied a wealth of details, but all are in 

 harmony with the general doctrine of the great master, which 

 may be summed up in the statement that what fundamentally 

 underlies each secretory process is the specific physiological activity 

 of the living substance l)y which the secreting surfaces are invested. 



After Schwanu had established the Cell Theory in 1839, and 

 it had been applied by Heule and Ivolliker to the physiology of 

 secretion, the idea of the living substance, as described by Johannes 

 Miiller in 1830, was more exactly conceived as the living epithelial 

 cells which clothe the internal cavities and the secreting surfaces. 

 Real knowledge of the intimate secretory processes of the gland 

 cells only became possible, however, after the progress of 

 histological technique enabled Heidenhain and his School to form 

 a morphological comparison between glands in the state of rest, 

 and those functioning actively. 



A more palpable advance in regard to the specific nature of 

 the secreting cells resulted from the chemical analyses of the 

 various products of their secretory activity, undertaken by a host of 

 observers. Advance in this direction has gone pari passu with 

 that of chemical physiology. Much, however, in regard to the 

 chemical composition of the secretions still remains incomplete 

 and imperfect, and what we know at present is little in comparison 

 with what remains to be learned except for certain secretions 



