PHYSIOLOGY OF ORGANS ' 153 



various phases of these two theories, hut the results are not entirely in 

 harmony, and a complete understanding of the elimination of wastes in 

 the kidney awaits the future. 



Secretion.— Every cell produces substances which are later used. 

 The elaboration of these substances is called secretion and the substances 

 elaborated are known as secretions. Unicellular organisms use most of 

 their secretions within the cell, as is true also of a large number of cells 

 of multicellular organisms; but there are cells which on the contrary 

 discharge their secretion. A secretion which is discharged may perform 

 its functions within a cavity of the body or may pass from the body. 

 Since excretions also may be discharged from the body, or in plant cells 

 may remain within the cell, it is not always easy to distinguish secretions 

 from excretions. In general, excretions are of no further value, while 

 secretions perform some function for the organism. 



The Process of Secretion. — Secretion occurs within a cell as a result 

 of the chemical activities of the general protoplasm, or perhaps of cer- 

 tain speciahzed structures within it. While the cytoplasm directly 

 performs the function of secretion, there is much evidence, some of which 

 has been given in Chapter III, that cells which have been deprived 

 of their nuclei do not secrete properly. This failure to carry on secretory 

 syntheses in the absence of the nucleus has been explained by one inves- 

 tigator by the assumption that the nucleus controls the rapid oxidative 

 processes of the cell. It is known that the process of secretion re- 

 quires considerable energy which is derived from oxidative processes. 

 The earliest visible traces of secretions are usually found near the nucleus 

 as minute granules or droplets which later move to the periphery of the 

 cell or to the region of discharge. Beyond these few facts little is known 

 with certainty regarding the process of secretion. 



Mode of Discharge. — ^Secretions are discharged from the cell in 

 various ways. A common mode is diffusion of liquid secretions through 

 the cell membrane. The secretions of the pancreas, Uver, and gastric 

 glands, and of some of the ductless glands escape from the cell in this 

 way. In some of these glands the secretions are stored within the cell in 

 granules which are changed to fluid form shortly before discharge. Some 

 secretions, such as oily secretions and mucus which is produced in 

 mucous cells in the form of mucin, are not diffusible. Mucin is discharged 

 through the open ends of the goblet cells, so called because of their shape 

 due to the enclosed mucin, or from other scattered cells which produce it. 

 In mucus glands the thin cytoplasmic border of the cells which are 

 inflated with mucin break down, thus permitting the escape of the secre- 

 tion. Cells producing oily secretions may break down at their free ends, 

 or in some instances the whole cell may disintegrate at the time of releas- 

 ing the secretion. 



In many glands the secretion is discharged from the cells and is 



