HUMAN STRUCTURE AND LIFE-ACTIVITIES 471 



395. Glands. The liver, pancreas, salivary glands, and 

 gastric glands have been mentioned as producers of special 

 secretions useful in digestion. Microscopic study shows that 

 a gland is composed of a layer of epithelial cells which rest 

 on a bed of connective tissue. In this latter tissua are 

 blood-vessels which supply food and oxygen to the cells 

 of the glands; and from the materials thus obtained the 

 cells manufacture their secretions, which they discharge 

 at the free end of the cells (i.e., opposite the end where they 

 may absorb from the blood). See Figs. 158, 159. 



(D) Sections of frog's stomach show structure of simple gland. 

 Pancreas sections show numerous tubes cut across at various angles. 



THE WORK OF THE DIGESTIVE ORGANS 



We have briefly considered the structure of the digestive 

 organs, and it is now our problem to study the work of these 

 organs, especially with reference to the preparation for ab- 

 sorption of the different kinds of food; i.e., the nutrients. 

 As in the frog, this digestion of foods is caused by secretions ; 

 and in the human body these are saliva from the salivary 

 glands, gastric juice from the glands in the wall of the stomach, 

 intestinal juice from the glands in the wall of the intestine, 

 pancreatic juice from the pancreas, and bile from the liver. 



396. Mechanical Processes in Digestion. Various move- 

 ments, due to the action of muscles in the digestive organs, 

 play two important parts : (1) in taking food and in mov- 

 ing it along through the digestive tube, and (2) in separat- 

 ing more or less solid food into small particles upon which the 

 digestive fluids can act rapidly. The muscular action of 

 the lips and jaws in taking and chewing (masticating) food 

 is so easily observed that no description is necessary. The 

 mastication process is important in that it results in mixing 

 saliva with the food (see 455, on hygiene of eating). 



After mastication comes the swallowing movements, 



