CHAPTER NINTH. 



OF THE SECRETIONS. 



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

 fluid is prepared from the food, for supplying 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 pur- 

 poses, 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 

 SECEETION. 



407. The organs by which these operations are performed 

 are much varied, consisting either of flat surfaces or mem- 

 branes, 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 surface of 

 the body is covered by them ; and they either discharge their 

 products directly upon the surface, as on the mucous mem- 

 brane, or they unite in clusters, and empty into a common 

 duct, and discharge by a sing e orifice, as is the case with 

 some of the intestinal glands, and of those from which the 

 perspiration issues from the skin. 



408. In the higher animals, where separate organs for 

 special purposes are multiplied, numerous sacs and tubes are 

 assembled into compact masses called glands. Some of these 

 are of large size, as the salivary glands, the kidneys, and 

 the liver. In these, clusters of sacs open into a common canal, 

 and this canal unites with similar ones, forming larger trunks ; 

 and finally, they all discharge by a single duct, as we find in 

 the salivary glands. 



409. By the organs of secretion two somewhat different 

 purposes are effected, namely, fluids of a peculiar character 

 are selected from the blood for important uses, such as the 

 saliva, tears, milk, &c., some of which differ but little in 

 their composition from that of the blood itself, and might be 



