110 THE BODY AT WORK 



donna) dilates the bloodvessels, increasing blood-pressure, but 

 it stops secretion. After belladonna-poisoning, the mouth, like 

 the skin, is hot and dry. Other drugs there are which provoke 

 a certain amount of secretion, even after the bloodvessels 

 going to the gland have been tied. It is possible, by stimulating 

 the chorda tympani, to obtain a pressure in the fluid in the 

 duct very much greater than that in the bloodvessels which 

 supply the gland. Here we have clear proof that secretion is 

 not filtration. Filtration is the passage of fluid through a 

 filter-bed from a higher to a lower pressure. In filtration, 

 moreover, soluble diffusible salts accompany the water. The 

 saliva contains only half as much of these diffusible salts as the 

 blood. Therefore the gland tissue stops half the salts. Secre- 

 tion is an active process carried out by the gland-cells, under 

 the influence of nerves, in opposition to the laws of filtration. 

 The gland-cells determine how much water shall pass through 

 them and what percentage of salts shall accompany the water. 

 How does a gland-cell make the substance which it secretes ? 

 There is no reason for supposing that the ptyalin or the mucus 

 which the salivary glands secrete is present in the blood, 

 either ready formed, or, as it were, half formed, in combinations 

 which can be easily broken up. All the evidence obtainable 

 points to the conclusion that the gland-cells take out of the 

 lymph proteid materials from which they manufacture the 

 peculiar substances which they secrete. During rest, granules 

 accumulate in the cells. During activity they disappear. It 

 has been shown in the case of the gastric glands that these 

 granules consist of the special ferment which the gland secretes, 

 in an inactive form. It may be that it is combined with a sub- 

 stance which prevents it from exerting its digestive action on 

 the cells within which it is made ; damped, as gunpowder is 

 damped during transit. Or it may be that it is not a finished 

 ferment ; it may need a further addition to its molecule. During 

 activity, while the granules disappear, proteins accumulate at 

 the bases of the cells, giving to a tube of gland-cells the appear- 

 ance of a peripheral non-granular zone. This proteid sub- 

 stance must have come from the lymph, and the inference 

 seems inevitable that the cells have taken into their protoplasm 

 a supply of material which will serve for the manufacture of 

 additional granules. Each gland-cell is therefore an indepen- 



