110 THE HUMAN MECHANISM 



occasionally to secure better contact of the digestive juice with the 

 material undergoing digestion. 



1. The white of soft-boiled (3-4 minutes) egg. This is composed 

 mostly of protein ; it will be dissolved. Into what is the egg white 

 changed ? 



2. A piece of tendon, which can be obtained from any butcher. 

 This is composed of the kind of fibers which are found in the con- 

 nective tissues holding the cells together (see Chap. III). The tendon 

 first swells, then gradually disintegrates, its protein (albuminoid, p. 94) 

 fibers going into solution. A small residue will be left. 



3. A piece of the lean of rare meat cut or chopped into small pieces. 

 The meat will disintegrate, owing to the solution of its connective tissue 

 fibers ; then the protein muscle fibers will go into solution, being changed 

 into soluble peptids. 



4. A piece of lean of well-cooked meat. The result will be much like 

 that in (3) except that it will probably take longer to bring the muscle 

 fibers into solution. 



5. Some jelly (made from gelatin) which has set. This will be 

 gradually dissolved. 



6. Some fat (not gristle) of beef. The mass will disintegrate for the 

 same reason as in the case of meat. The fat itself will be unacted on, 

 but will rise to the top, where it may form a layer of fat or oil. 



7. A piece of bread. This consists of starch, fat, etc. held togethei 

 by the tenacious gluten (a protein). As the gluten is dissolved by the 

 gastric juice the undissolved starch, fat, etc. is set free. 



8. Some starch paste. No action. 



9. Some fried steak. Note the prolongation of the period of digestion. 



Instructive experiments may also be made with cheese, 

 sweetbreads, potatoes, peas, etc. They would all bring out 

 the main points in the action of the gastric juice. These 

 may be summed up as follows: Gastric juice has no effect 

 upon pure fats (although it plays an important part in the 

 digestion of adipose tissue 1 ), nor upon carbohydrates, such 

 as starch or sugar. Its part in digestion consists in its action 



1 The fat of meat consists of connective tissue whose cells are greatly 

 swollen with drops of fat. In typical adipose tissue the connective-tissue 

 cell becomes one large fat droplet surrounded by the thin layer of the cell 

 cytoplasm with its nucleus. These ff fat cells," like the muscle fibers of 

 meat, are thus held together by the fibers of connective tissue and are set 

 free when the latter are digested and dissolved away by the gastric juice 

 (see Figs. 90-92). 



