Tubule of 

 gland ▲ 



Drop of 



secretion 



I (B i o ) o ' < . mnitlini 



Tubule of glan 



Ghnd - cell secretion 



N 



Lymph 



■='^=^_^ Capillary 



^Food material 



Blood vessels 



HOW A GLAND SECRETES 



a.f: 



Materials are transformed in a gland by chemical action in the epithelial, or lining, 

 cells. The raw materials are derived from the blood stream or the lymph. The 

 specific substance formed by the gland is diffused out of the epithelial cells into the 

 tube or pit which they surround. The secreted substance is discharged from the gland 

 through a duct, or little tube. The excretions of the specific secreting cells are re- 

 moved by osmosis into the lymph or blood, as in the case of other body cells 



Glands and Juices We have seen that the carbohydrates, fats and pro- 

 teins are split into simpler compounds by specific ferments in the juices 

 secreted by glandular organs. But there are many sugars and many fats and 

 many proteins. Among the enzymes secreted by glands in the walls of the 

 small intestine, some convert sucrose and other complex sugars into simpler 

 ones. A certain enzyme will split one sugar, but will have no effect what- 

 ever on another sugar. Proteins, when digested, break first into proteoses, 

 then into peptones, then into numerous peptids, until finally only many 

 kinds of amino-acids are left. At each stage in the cleavage of a protein into 

 the fifteen or more amino-acids, a special enzyme operates. 



There are many kinds of glands besides those which produce digestive 

 juices. For example, the tears come from special glands, as do sweat, milk, 

 the mucus. The shell of an oyster may be considered as a precipitated lime 

 secreted by skin cells acting very much like glands. The kidneys are really 

 large glands which remove wastes from the blood, making them into urine, 

 and then discharge the urine through special ducts (see page 218). Still 



169 



