326 A MANUAL OF PHYSIOLOGY 



ducts as fast as it is formed. But it should be mentioned that some 

 observers deny that the acid is secreted in the depths of any cell 

 from the chlorides of the blood, and believe that it is formed at the 

 surface of contact of the stomach-wall with the gastric contents from 

 the sodium chloride of the food by an exchange of sodium ions (p. 362)1 

 for hydrogen ions from the blood or lymph. They point out in 

 favour of this view that when, instead of sodium chloride, sodium 

 bromide is given in the food, the hydrochloric acid in the stomach- 

 is to a large extent replaced by hydrobromic acid. And they argue 

 that this cannot be due to the decomposition of the bromide by- 

 hydrochloric acid, since it occurs in animals deprived for a consider- 

 able time of salts, and in ' salt-hunger ' the stomach contains no acid 

 (Koeppe). It may be, however, that even in ' salt-hunger ' the pres- 

 ence of sodium bromide in the stomach stimulates the secretion of 

 hydrochloric acid, which then decomposes the bromide, with the 

 formation of hydrobromic acid. The sodium chloride formed in 

 the double decomposition might be re-absorbed, and the stock of 

 chlorides in the blood remain undiminished. There are, besides, 

 weighty theoretical objections to this hypothesis. On the other 

 hand, it has been shown that mechanical stimulation of the stomach 

 in a fasting individual, by a kind of rotatory sound, causes hydro- 

 chloric acid to appear in considerable amount, even when the viscus 

 has been previously washed out so as to remove all the chlorides 

 (Wesener). In this case the acid must be formed from the chlorides 



of the blood. 



The rennet ferment of the gastric juice is formed in the chief cells, 

 and has a precursor or zymogen like the other digestive ferments. A 

 glycerine or watery extract of the salivary glands always contains 

 active amylolytic ferment, if the natural secretion is active. So that 

 if ptyalin is preceded by a zymogen in the cells, it must be very easily 

 changed into the actual ferment. 



The Quantitative Estimation of Ferment Action. Since we have 

 as yet no certain method of freeing ferments from impurities, our only 

 quantitative test is their digestive activity. And since a very small 

 quantity of ferment can act upon an indefinite amount of material 

 if allowed sufficient time, we can only make comparisons when the 

 time of digestion and all other conditions are the same. If we find 

 that a given quantity of one gastric extract, acting on a given weight 

 of fibrin, dissolves it in half the time required by an equal amount 

 of another gastric extract, or dissolves twice as much of it in a given 

 time, we conclude that the digestive activity of the pepsin is twice 

 as great in the first extract as in the second, or, as it is sometimes 

 more loosely put, that the one contains twice as much pepsin as the 

 other. A convenient method of estimating the rate at which the 

 fibrin disappears is to use fibrin stained with carmine. As solution, 

 goes on, the dye colours the liquid more and more deeply, and by 

 comparing the depth of colour at any time with standard solutions 

 of carmine, we get the quantity of dye set free, and therefore of fibrin* 

 digested. This method cannot be used for trypsin. As a test of the 

 activity of a diastatic ferment, we take the amount of sugar formed in 

 a given time in a given quantity of a standard starch solution. 



