SECRET I OX. 215 



simply as a passive membrane. This theory has not proved entirely acceptable 

 for various reasons. It has been shown that living membranes offer consider- 

 able 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 otherwise, 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 pas- 

 sive filters. In glands like the kidney, and in other glands as well, it lias 

 not, as yet, been shown conclusively that the amount of water and salts 

 increases 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 urine to an extent that it is difficult to explain by the use of the 

 filtration and diffusion theory alone. 



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 w r e are justified in assuming that these processes do actually occur 

 and serve 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 

 theories based on these factors alone do not suffice to explain all the phe- 

 nomena connected with the secretion of water and salts. Until the contrary is 

 definitively proved we may suppose that the epithelial cells are actively con- 

 cerned 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 witli 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. — Heidenhain recognized two types 

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

 character of the secretion and upon the histological appearance of the secreting 

 cells. The classification as originally made was applied only to the salivary 

 glands and to similar glands found in the mucous membranes of the mouth 



1 Mittheil. mm physiol. Lab. des Octroi, med.-chir. Tnsliliiis in Storkholm, lSSo. 



