306 PRACTICAL PHYSIOLOGY 



CHAPTER XXI 

 DIGESTIVE SECRETIONS 



Estimation of Diastatic Activity. For the accurate determination of 

 the action of ptyalin (or any other amylolytic ferment) on starch one 

 may estimate the reducing power of the incubated solution after a 

 certain time. Besides being tedious, this method is uncertain, because 

 of the different reducing powers of maltose and dextrose, both of 

 which sugars frequently result by salivary digestion, especially when 

 this is prolonged. 



A simpler and more serviceable method depends on the colour reaction 

 of starch with iodine, and is conducted as follows : 



Prepare a 1 per cent, starch paste solution, 1 and place the beaker 

 containing it in ice water. Collect some saliva and dilute 1 c.c. of it to 

 10 c.c. with distilled water, and filter. Take a series of five test tubes 

 labelled A, "B, C, etc., and with a 1 c.c. pipette graduated in 100 parts 

 deliver into tube A 1 c.c. of the diluted saliva ; into B 0-75 c.c. ; into 

 C 0-5 ; into D 0-25 ; and into E 0-1. 



Place the tubes in a beaker containing ice water, and then deliver 

 into each 5 c.c. of 1 per cent, cooled starch solution. The cold prevents 

 any ferment action until all are ready. Now remove the tubes to 

 another beaker containing w T ater at 40 C., and gently shake them so 

 that the contents become thoroughly mixed. Note the exact time at 

 which the tubes are placed in the warm water. At the end of half an 

 hour remove the tubes simultaneously to ice water, and shake them 

 gently so as to ensure thorough cooling. Fill each tube to within half 

 an inch of the top with distilled water and add a few drops of iodine 

 solution. 2 Close each tube with the finger and invert so as to mix. 

 It will be seen that there is a gradation of colours in the different 

 tubes from blue through violet and brown to yellow. Note the tube 

 which just shows a bluish tint. The next one higher up in the series 

 is taken as that in which all starch has just disappeared. From the 

 amount of diluted solution added to this, calculate the amount of 

 undiluted saliva required to convert 100 c.c. of 1 per cent, starch 

 solution into dextrines in half an hour at 40 C. Thus, suppose that 

 the tube containing 0-25 c.c. diluted saliva is found to be that which 

 just shows a bluish tint. In the next (viz. containing 0-5 c.c. saliva) 



all the starch has disappeared, therefore -y^- = -05 c.c. saliva can 



hydrolyse 5 c.c. 1 per cent, starch, or 1 c.c. can invert 100 c.c. 1 per cent, 

 starch. The diastatic action of pancreatic juice, of liver extract, of 

 blood serum or of malt diastase may be measured in the same way, 

 but different amounts of the ferment solution must be employed. 8 



1 Weigh 1 or 2 gms. of pulverised " soluble starch," and stir it up in a 

 beaker with an amount of distilled water sufficient to make a 1 per cent, 

 solution. Place on a boiling waterbath and continue stirring until a clear 

 opalescent solution is obtained. Cool before using. 



2 Care must be taken that sufficient iodine solution is added to give the 

 maximal reaction, but an excess must be avoided. The iodine solution is 

 made by dissolving 12-7 gr. iodine in water containing 25 gr. potassium 

 iodide, and then diluting to 1,000 c.c. 



3 Care must be taken when using organ extracts, such as those of liver, 



