194 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



pure "starch-blue" colour has just definitely changed and a very 

 slight tinge of violet is perceptible. From the position of this tube 

 in the rack the time required to produce this effect is obtained. It 

 is a measure of the activity of the enzyme. 



If the digestion time is much less than 10 minutes, repeat the 

 experiment with a diluted enzyme, attempting to arrive at a diges- 

 tion period of about 10 minutes. Thus, if in the first experiment 

 the third tube shows the change, dilute 3 cc. of the enzyme with 

 7 cc. of distilled water. If the fourth tube shows the change, dilute 

 4 cc. with 6 cc. of distilled water, and so on. In the second experi- 

 ment it is advisable to take a sample drop every half-minute when 

 near the expected end point. These "half-minute " tubes can be 

 subsequently identified by placing them behind the " minute " 

 tubes in a double test-tube rack. 



NOTES. i. The drops of iodine should not be added to the distilled 

 water in the tubes until just before (or even after) the enzyme has been 

 added to the starch tube. It is important to add only one drop to each 

 tube. 



2. The author's " Distributor " (fig. 29) is convenient for measuring 

 the 3 cc. of distilled water. 



3. The effect of the concentration of hydrogen-ions can be investigated 

 by varying the P H of the buffer solution added (see Ex. 240). 



4. The author suggests that the unit of amylase should be taken as the 

 amount which can convert TO cc. of 2 per cent, soluble starch to the stage 

 obtained in 10 minutes. So if 2 cc. of a 4 in 10 dilution effects the change in 



10 10 

 9-5 mins., then 2 cc. contains - x = 2-62 units. This can be expressed, 



A = 131 per 100 cc. 



C, Gastric Juice. 



Human gastric juice has been obtained in certain cases 

 in which an artificial opening into the stomach has been 

 necessitated owing to stricture of the oesophagus. It con- 

 tains about 0-37 per cent, of hydrochloric acid, small 

 amounts of the chlorides of sodium, calcium and potassium, 

 traces of phosphates and of mucin, together with the 

 enzymes, pepsin, rennin and lipase. 



From a practical standpoint the composition of the 

 gastric contents at stated times after the ingestion of 



