CHAPTER XVIII 

 EXTERNAL SECRETIONS 



Secretion is a term applied to a process by which complex fluids are 

 formed from the constituents of the lymph which are separated from the 

 blood-stream by the activities of the endothelial cells of the capillary wall, 

 as the blood flows through the capillary blood-vessels. In this process the 

 endothelial cell is aided by the physical forces, diffusion, osmosis, and filtra- 

 tion. These separated materials may be utilized in several ways: 



1. For the repair of the tissues, for growth, for the liberation of energy. 



2. For the elaboration or production by specialized organs of a variety of 



complex fluids and specific materials of widely different application. 

 The fluids and specific materials thus formed are utilized for the most 

 part to meet some special need of the body. All such fluids and mate- 

 rials are termed secretions, and the organs by which they are formed are 

 termed secretor organs. Secretions whether simple or complex may in 

 a general way be divided into two groups, viz. : external and internal. 

 External Secretions. An external secretion may be defined as a more 

 or less complex fluid formed by the secretor activities of epithelial cells of 

 glands, which is discharged through well-defined ducts on the surfaces of 

 the body, the skin or mucous membrane. The glands by which they are 

 formed or secreted are known as glands of external secretion. 



Internal Secretions. Internal secretions may be defined as more or 

 less complex materials or agents formed by the activities of epithelial cells of 

 organs, and which are discharged into, and distributed by the blood to organs 

 and tissues near and remote, the activities of which they influence in varying 

 ways and degrees. The glands by which they are formed or secreted are 

 known as glands of internal secretion. 



Organs of External Secretions. All organs belonging to this group 

 consist primarily of a thin delicate homogeneous membrane, one side of 

 which is covered with a layer of epithelial cells and the other side of which 

 is closely invested by a network of capillary blood-vessels, lymph-vessels, 

 and nerves. Though the epithelial cells have a general histologic resem- 

 blance one to another, their physiologic function varies in different situations, 

 in accordance probably with their ultimate chemic structure, a fact which 

 determines the difference in the character of the secretions. 



These organs may consist of a single layer of cells or a group of cells, and 

 may be subdivided into 



1. Secreting membranes. 



2. Secreting glands. 



The secreting membranes are the mucous membranes lining the 

 gastro-intestinal, the pulmonary, and the genito-urinary tracts, and the 

 serous membranes lining closed cavities, such as the pleural, pericardial, 

 peritoneal, and synovial membranes. 



The mucous membranes are soft and velvety in character and are com- 



