lg THE VITAL PROCESSES 



derived from the lymph, the cells carry on three well- 

 defined processes, known as absorption, assimilation, and 

 excretion. 



Absorption is the process of taking water, food, and 

 oxygen into the cells. 



Assimilation is a complex process which results in the 

 addition of the absorbed materials to the protoplasm. 

 Through assimilation the protoplasm is built up or renewed. 



Excretion is the throwing off of such waste materials as 

 have been formed in the cells. These are passed into the 

 lymph and thence to the surface of the body. 



Absorption, assimilation, excretion, and also reproduc- 

 tion are performed by all classes of cells. They are, on 

 this account, referred to as the general work of cells. 



The Special Work of Cells. In addition to the general 

 work which all cells do in common, each class of cells in 

 the body is able to do some particular kind of work a 

 work which the others cannot do or which they can do 

 only to a limited extent. This is spoken of as the special 

 work of cells. Examples of the special work of cells are 

 found in the production of motion by muscle cells and in 

 the secretion of liquids by gland cells. It may be noted 

 that while the general work of cells benefits them individ- 

 ually, their special work benefits the body as a whole. 

 Another example of the special work of cells is found in 

 the 



Production of the Intercellular Material. Though most 

 of the cells of the body deposit to a slight extent this 

 material, the greater part of it is produced by a single class 

 of cells found in bone, cartilage, and connective tissue. 

 Cartilage, bone, and connective tissue differ greatly from 

 the other tissues in the amount of intercellular material 

 which they contain, the difference being due to these cells. 



