PRIMARY FUNCTIONS OP THE ORGANISM 41 



Secretion. - - The majority of cells retain in greater 

 or less degree the primary function of protoplasm 

 called secretion. By virtue of its chemical and 

 physical organization living matter not only builds 

 non-living food substance into itself to form new 

 protoplasm (nutrition), but it reverses the process 

 and forms from its living substance non-living prod- 

 ucts or secretions. Certain types of cells are spe- 

 cialized in this direction to a great degree, and we speak 

 of them as secretory or gland cells. But many animal 

 cells and nearly all plant cells secrete a denser sub- 

 stance about themselves, the cell-wall, and further, 

 a cement substance which binds the cells together into 

 a unified mass. Cellular secretions are thus of two 

 sorts, " permanent " secretions that remain in place 

 after being formed, and secretions which, when 

 elaborated, are passed out of the cell-body to other 

 parts of the organism. In plant tissues the cell-wall 

 is usually greatly thickened and strengthened by the 

 secretion of cellulose, a derivative of which gives 

 wood its hard quality. In animal tissues the cell- 

 wall is not usually so thickened, but a similar result 

 is obtained by the development of intercellular 

 substances. These give the connective tissues their 

 characteristic structure and qualities. In the latter 

 the cells are specialized in the production of secre- 

 tions until the functions of contractility, irritability, 

 and conduction have almost if not entirely dis- 

 appeared, and the intercellular substance is produced 

 in such quantities as to outbulk many times the 

 living cells themselves. According to the nature 



