156 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ing in chemical composition, it would seem inevitable that water should filter 

 through the membrane and that processes of osmosis would be set up, further 

 changing the nature of the secretion. Upon this theory the water and salts in 

 all secretions were regarded merely as trausudatory products, and so far as they 

 were concerned the epithelium was supposed to act simply as a dead membrane. 

 This theory has not proved entirely acceptable for various reasons. It has 

 been shown that living membranes offer considerable resistance to filtration 

 even when the liquid pressure on one side is much greater than on the other. 

 Tigerstedt 1 and Santessen, for instance, found that a lung taken from a frog 

 just killed gave no filtrate when its cavity was distended by liquid under a 

 pressure of 18 to 20 centimeters, provided the liquid used was one that did 

 not injure the tissue. If, however, the lung-tissue was killed by heat or other- 

 wise, filtration occurred readily under the same pressure. In some glands, 

 also, the formation of the water and salts, as has been said, is obviously under 

 the control of nerve-fibres, and this fact is difficult to reconcile with the idea 

 that the epithelial cells are merely passive filters. In glands like the kidney, 

 and in other glands as well, it has been shown that the amount of water and 

 salts does not increase in proportion to the rise of blood -pressure within the 

 capillaries, as should happen if filtration were the sole agent at work, and 

 furthermore, certain chemical substances when injected into the blood may 

 increase the flow of water in the secretion to an extent that cannot be well 

 accounted for in any other way than by supposing that they act as chemical 

 stimuli to the epithelial cells. 



While, therefore, it cannot be denied that the anatomical conditions pre- 

 vailing in the glands are favorable to the processes of filtration and osmosis, 

 and while no one is justified in denying that these processes do actually occur 

 and seem to account in part for the appearance of the water and inorganic 

 salts, it seems to be clear that in the present condition of our knowledge these 

 factors alone do not suffice to explain all the phenomena connected with the 

 secretion of water and salts. We must suppose that the epithelial cells are 

 actively concerned in the process. The way in which they act is not known ; 

 various hypotheses have been advanced, but none of them meets all the facts 

 to be explained, and at present it is customary to refer the matter to the vital 

 properties of the cells that is, to the peculiar physical or chemical properties 

 connected with their living structure. 



We may now pass to a consideration of the facts known with regard to the 

 physiology of the different glands considered merely as secretory organs. 

 The functional value of the secretions will be found described in the sections 

 on Digestion and Nutrition. 



B. Mucous AND ALBUMINOUS (SEROUS) TYPES OF GLANDS ; SALIVARY 



GLANDS. 



Mucous and Albuminous Glands. Heideuhain recognized two types 

 of glands, the mucous and the albuminous, basing his distinction upon the 



1 Mittheil. vom physiol. Lab. des Carol, med.-chir. Institute in Stockholm, 1885. 



