GLANDS AND THE WORK OF EXCRETION 



199 



ilar to that of a bunch of grapes. The general structure 

 of the different kinds of glands is shown in Fig. 85. 



FIG. 85. Diagram illustrating evolution of glands. A. Simple secret- 

 ing surface, i. Gland cells. 2. Basement membrane. 3. Blood vessel. 

 4. Nerve. B. Simple tubular gland. C. Simple saccular gland. D. Com- 

 pound tubular gland. E. Compound saccular gland. F. A compound race- 

 mose gland with duct passing to a free surface. G. Relation of food canal 

 to different forms of glands. The serous coat has a secreting surface. 



Nature of the Secretory Process. At one time the gland 

 was regarded merely as a kind of filter which separated 

 from the blood the ingredients found in its secretions. 

 Recent study, however, of several facts relating to secre- 

 tion has led to important modifications of this view. The 

 secretions of many glands are known to contain substances 

 that are not found in the blood, or, if present, are there in 

 exceedingly small amounts. Then again the cells of cer- 

 tain glands have been found to undergo marked changes 

 during the process of secretion. If, for example, the 



