HUMAN STRUCTURE AND LIFE- ACTIVITIES 473 



a rule, the large masses of solid food are held back in the 

 stomach. 



The movements of the intestine consist of constrictions. 

 These sometimes appear in series at certain points, and then 

 next at the points which are halfway between the first con- 

 strictions. This causes a thorough mixing of the foods with 

 the digestive fluids. Another form of constriction which 

 occurs at intervals moves along the intestine and forces the 

 contents towards the large intestine. The above experiment 

 with rubber tubing illustrates this type of constriction. Such 

 movements are known as peristaltic contractions. 



397. Why Foods must be Digested. We have seen in 

 our study of the frog that foods must be prepared so that 

 they may be absorbed through the lining membranes (epi- 

 thelium) of the digestive organs into the blood. Jn other 

 words, foods must be capable of osmosing through mem- 

 branes ; and most foods, as taken into the mouth, are not 

 ready for this process. Sugar and common salt dissolved 

 in water are practically the only cases of common food which 

 can osmose or be absorbed without digestion. Even milk, 

 our most common liquid food, contains droplets of fat too 

 large for absorption and which must be dissolved in prepara- 

 tion for absorption. 



(D) Microscopic Examination of Milk. Mount a drop of milk on 

 a glass object-slide. Examine with (1) low-power, (2) high-power 

 objective. Note the droplets of fat. These are lighter than the 

 water of the milk ; and, like oils in general, rise to the surface when 

 milk stands, forming the concentrated layer of fat which we call 

 cream. When the cream is shaken or agitated, as in a churn, these 

 fat droplets fuse together into larger masses of fat, which we call 

 butter. 



Since digestion practically means the preparation of foods 

 for absorption, it is convenient to study the action of the 

 various digestive secretions (saliva, gastric juice, bile, and 

 pancreatic fluid) by adding them to foods in test-tubes ; and 



