343 



Or a piece of a glass capillary-tube filled with heat-coagulated egg-white 

 may be cut off and placed in the digestive mixture (Mett's tubes). At 

 the end of the period of digestion the length of the piece of tube and 

 that of the undigested remnant of the column of coagulated protein 

 are measured with a millimetre scale under a low- power microscope. 

 The difference gives the length of the column digested. If i c.c. of 

 gastric juice caused in a given time digestion of 2 mm. of the egg-white, 

 4 c.c. of the same juice would digest in the same time and under identical 

 conditions about 4 mm., and 9 c.c. about 6 mm. 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. To 

 determine the activity of a liquid, say, the pancreatic juice, as regards 

 fat-splitting ferment, the acidity of the emulsion formed by the juice 

 and fat after standing for a definite time at, a given temperature (with 

 occasional shaking) can be estimated by titration with baryta solution. 



In addition to the ferments of the digestive juices which act extra- 

 cellularly in the lumen of the alimentary canal, and those which 

 do their work intracellularly in its walls, micro-organisms are 

 present in the gut, and even in normal digestion contribute to the 

 changes brought about in the food ; while under abnormal conditions 

 they may awaken into troublesome, and even dangerous, activity. 

 It is now known that these act by producing intracellular enzymes. 



It may be noted here, although the subject must be again referred 

 to (p. 390), that specific substances capable of inhibiting the action 

 of ferments exist. Some of these antiferments are normally present 

 in the body an antitrypsin, for instance, in normal blood-serum. 

 Numerous antiferments may be artificially obtained by immunizing 

 animals with the original ferments. Thus an antilipase is found 

 in the serum of rabbits after injection of pancreatic lipase, and an 

 antiemulsin after injection of emulsin. Injection of rennin causes 

 the formation of antirennin, which can be demonstrated in the 

 blood-serum and milk of the immunized animal. Besides the anti- 

 ferments, bodies, sometimes spoken of as ' co-enzymes, ' are known 

 which aid the action of certain enzymes, not in the general way 

 in which, for instance, increase of temperature up to the optimum 

 does, but in some quite special manner. Thus, as we shall see, 

 bile salts greatly facilitate the fat-splitting action of lipase. This 

 co-operation is not to be confounded with the activation of the 

 proferment or zymogen which in many cases represents the inactive 

 form of the enzyme, while it is still within the secreting cells. For, 

 once activated, the fully formed enzyme cannot be made to revert 

 to the zymogen stage. For example, the active trypsin of the 

 pancreatic juice cannot be changed into inactive trypsinogen, 

 whereas substances which simply co-operate or co-act with enzymes 

 leave the latter unaltered when they are removed. Thus lipase does 

 not preserve the increased activity conferred upon it by bile salts 

 when the bile salts are again separated from the digestive mixture. 



It is now necessary to consider in detail the nature of the various? 



