us HOW FOOD IS PREPARED FOR BODY USES 



Mouth of gland 



Blood vessel 



(par|'kre-as) and the liver, both connected with the small intestine 

 by ducts, and the intestinal glands in the walls of the small intes- 

 tine. Besides these glands which aid directly in digestion, there 

 are several others known as the endocrine (en'd6-ki'in) or ductless 

 glands, because they have no ducts or tubes to carry off their 

 contents. These glands give theii' secretions, which contain sub- 

 stances known as hor'mcrnes, dhectly uito the blood. We shall 

 study theii' functions later. 



Structure of Glands. — In its simplest form a gland may be a 

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

 ture and pour out a substance known as a secretion. The nectar 



gland of a flower is such a collection 

 of ceUs. In animals, glands are usu- 

 ally tubular, such as the gastric gland 

 shown on page 153, or like little sacs, 

 as illustrated on this page. In aU 

 animal glands there is a rich blood 

 supply to and from the secreting ceUs 

 that line the tubes or sacs, and tiny 

 nerves which control the gland ceUs 

 and blood supply. 



Enzymes and their Work. — Certain 

 gland cells form secretions containing 

 the wonderful chemical activators called 

 enzymes, which we have ah'eady found 

 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 ceU, although 

 the}^ are not themselves ahve. We do not laiow much about 

 enzymes themselves, but we can observe w^hat the}^ do. Some 

 enzymes render soluble different foods, others work in the blood, 

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

 to oxidation, when work is done. Enzymes are ver^^ 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 act long in the 



Diagram of a gland. 



