362 



HOW FOOD IS PREPARED FOR BODY USE 



"blood 



i~ _; /C.-Cotpillary 



Structure of glands. In its simplest form a gland may be a 

 collection of cells which, by means of their own activity, manu- 

 facture and pour out a substance known as a secretion. The 

 nectar gland of a flower is such a collection of cells. In animals, 

 glands are usually tubular, such as the gastric gland shown on 

 page 371, or like little sacs, as in the diagram. In all animal 

 glands there is a rich blood supply to and from the secreting cells 



it , that line the tubes or sacs, and 



mouth of . 



planet tiny nerves which control the 



gland cells and blood supply. 



Enzymes and their work. 

 Certain gland cells form secre- 

 tions containing the chemical 

 activators called enzymes, which 

 we have already learned cause 

 digestion in plants. In animals 

 the enzymes secreted by the 

 cells of the glands and poured 

 out into the food tube act upon 

 insoluble foods so as to change 

 them to a soluble form. They 

 are the product of the activity 

 of the cell, although they are 

 gIand not themselves alive. We do 

 not know much about enzymes 

 themselves, but we can observe what they do. Some enzymes 

 render certain foods soluble, others work in the blood, and 

 still others probably act within the cells of the body as an aid 

 to oxidation, when work is done. Enzymes are very sensitive 

 to changes in temperature and to the degree of acidity or 

 alkalinity of the material in which they act. We shall find that 

 the enzymes from glands in the walls of the mouth will not be 

 active very long in the stomach after the change from the alkaline 

 medium in the mouth to the acid medium in the stomach. En- 

 zymes seem to be able to work indefinitely, provided the surround- 

 ings are favorable. A small amount of digestive enzyme, if it 

 had long enough to work, could digest a large amount of food. 



lvraph spaa 



A gland cut lengthwise. Explain how 

 might obtain its secretions. 



