214 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ever, which influenced physiologists in coming to this conclusion are as 

 follows : 



Microscopic examination has demonstrated clearly that in many cases parts 

 of the epithelial cell-substance can be followed into the secretion. In the 

 sebaceous secretion the cells seem to break clown completely to form the mate- 

 rial of the secretion ; in the formation of mucus by the goblet cells of the 

 mucous membrane of the stomach and intestines a portion of the cytoplasm 

 after undergoing a mucoid degeneration is extruded bodily from the cell to 

 form the secretion ; in the mammary glands a portion of the substance of the 

 epithelial cells is likewise broken off and disintegrated in the act of secretion, 

 while in other glands the material of the secretion is deposited within the cell 

 in the form of visible granules which during the act of secretion may be 

 observed to disappear, apparently by dissolution in the stream of water passing 

 through the cell. Facts like these show that some at least of the products of 

 secretion arise from the substance of the gland-cells, and may be considered as 

 representing the results of a metabolism within the cell-substance. From 

 this standpoint, therefore, we may explain the variations in the organic 

 constituents of the secretions by referring them to the different kinds of 

 metabolism existing in the different gland-cells. The existence of distinct 

 secretory nerves to many of the glands is also a fact favoring the view of 

 an active participation of the gland-cells in the formation of the secretion. 

 The first discovery of this class of nerve-fibres we owe to Ludwig, who (in 

 1851) showed that stimulation of the chorda tympani nerve causes a strong 

 secretion from the submaxillary gland. Later investigations have demon- 

 strated the existence of similar nerve-fibres to many other glands — for 

 example, the lachrymal glands, the sweat-glands, the gastric glands, the 

 pancreas. Recent microscopic work indicates that the secretory fibres end in 

 a fine plexus between and around the epithelial cells, and we may infer from 

 this that the action of the nerve-iinpulses conducted by these fibres is exerted 

 directly upon the gland-cells. 



The formation of the water and inorganic salts present in the various 

 secretions offers a problem the general nature of which may be referred to 

 appropriately in this connection, although detailed statements must be reserved 

 until the several secretions are specially described. The problem involves, 

 indeed, not only the well-recognized secretions, but also the lymph itself as 

 well as the various normal and pathological exudations. Formerly the occur- 

 rence of these substances was explained by the action of the physical processes 

 of filtration, diffusion, and osmosis through membranes. With the blood under 

 a considerable pressure and with a certain concentration in salts on one side 

 of the basement membrane, and on the other a Liquid under low pressure and 

 differing in chemical composition, it would seem inevitable that water should 

 filter through the membrane and that processes of osmosis and diffusion should 

 be set up, further changing the nature of the secretion. Upon this theory the 

 water and salts in all secretions were regarded merely as transudatory prod- 

 ucts, and so far as they were concerned the epithelium was supposed to act 



