CHAPTER NINTH. 



OF THE SECRETIONS. 



261. WHILE, by the process of digestion, a homogeneous 

 fluid is prepared from the food, and supplies new material to 

 the blood, another process is also going on, by which the 

 blood is analyzed, as it were ; some of its constituents being 

 selected and so combined as to form products for useful 

 purposes, while other portions of it which have become useless 

 or injurious to the system are taken up by different organs, 

 and expelled in different forms. This process is termed 

 SECRETION. 



262. The organs by which these operations are per- 

 formed are much varied, consisting either of flat surfaces or 

 membranes, of minute 'simple sacs, or of delicate elongated 

 tubes, all lined with minute cells, called epithelium cells, 

 which latter are the real agents in the process. Every sur- 

 face of the body is covered by them, and they either dis- 

 charge their products directly upon the surface, as on the 

 mucous membrane, or they unite in clusters and empty into 

 a common duct, and discharge by a single orifice, as is the 

 case with some of the intestinal glands, and of those from 

 which the perspiration issues upon the skin, (Fig. 94.) 



