318 A MANUAL OF PHYSIOLOGY . 



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. 



Besides 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 con- 

 ditions they may awaken into troublesome, and even dangerous, 

 activity. It is now known that many of these act by pro- 

 ducing intracellular enzymes. 



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

 referred to (p. 361), 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 immunising 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 anti- 

 rennin, which can be demonstrated in the blood-serum and milk 

 of the immunized animal. 



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

 various juices yielded by the digestive glands, and the mechanism 

 of their secretion. Since it is along the digestive tract that 

 glandular action is seen on the greatest scale, this discussion 

 will practically embrace the nature of secretion in general. 

 And here it may be well to say that, although in describing 

 digestion it is necessary to break it up into sections, a true 

 view is only got when we look upon it as a single, though com- 

 plex, process, one part of which fits into the other from beginning 

 to end. It is, indeed, the business of the physiologist, wherever 

 it is possible to insert a cannula into a duct and to drain off an 

 unmixed secretion, to investigate the properties of each juice 

 upon its own basis ; but it must not be forgotten that in the 

 body digestion is the joint result of the chemical work of five or 

 six secretions, the greater number of which are actually mixed 

 together in the alimentary canal, and of the mechanical work of 

 the gastro-intestinal walls. 



