224 



NUTRITION 



the microscope or by chemical analysis. Obviously the active produc- 

 tion of a new substance by the protoplasm of a secreting cell necessitates 

 some degree of loss in that protoplasm, for the time being at least. On 

 the basis of the degree of this destruction of the epithelial substance we 

 may distinguish three types of secretion proper: In the first, the most 

 common, the protoplasm is not obviously lessened, the preliminary 

 product (zymogen) appearing as granules, etc., within the cell-body. 

 In the second type the upper part of the cell-body becomes in a mass the 



FIG. 118 



M.B. 



Diagram of a gland: M.B., basement-membrane; E, epithelium; D, gland-duct; N.C., nerve- 

 center; M, mental influence; S, "sensory" influence from the basement-membrane; N, direct 

 influence on the cell-protoplasm; V.M., vaso-motor influence; A, artery; V, vein. (Lecture-chart.) 



secretory product; of this, milk-secretion is the best example. In the 

 third sort of secretion the whole cell-body passes off as the product; 

 mucous secretion is the type of this, and sebum the other chief instance. 

 As hinted above, many substances are given out by epithelium that do 

 not appear to view at all. These pass out of the protoplasm by osmosis 

 or diffusion, and are crystalloids, gases, or, rarely perhaps liquids. 

 About these invisible secretory phenomena, purely metabolic in nature, 

 little is known as yet. The secretion-products with which we are now 



