Introduction 



Cells may be described also with reference to their func- 

 tions or location or the tissue in which they are found, 

 as blood cells or corpuscles (Figs. 2 and 67), nerve cells 

 (Fig. 3), and connective-tissue cells. 



20. Vital Properties of Cells. Each cell has a life of its 

 own. It manifests its vital properties in that it is born, 

 grows, multiplies, decays, and at last dies. During its life 

 it assimilates food, works, 

 rests, and is capable of 

 spontaneous motion and 

 frequently of locomotion. 

 Some cells live a brief life 

 of 12 to 24 hours, as is 

 probably the case with 

 many of the cells lining 

 the alimentary canal; 

 others may live for years, 

 as do the cells of cartilage 

 and bone. 



A most interesting 



FIG. 4. Various Forms of Cells. 



A, columnar cells found lining various parts 

 of the intestines (called columnar epithe- 

 lium) ; B, cells of a fusiform or spindle 

 shape found in the loose tissue under the 

 skin and in other parts (called connective- 

 tissite cells) ; C, cell having many processes 

 such are found in some kinds of connec- 

 tive tissue ; D, primitive cells composed of 

 protoplasm with nucleus, and having no cell 

 wall. All are represented about 400 times 

 their real size. 



power of the living cell 



is motion, a beautiful form of which is found in ciliated 

 epithelium (sec. 23). Cells may move actively and pas- 

 sively. In the blood the corpuscles are swept along by 

 the current, but some of the white corpuscles are able to 

 make their way actively through the tissues, as if guided 

 by some sort of instinct. They protrude a portion of their 

 protoplasm in the direction in which they wish to move and 

 then draw the rest of the body towards this "pseudopodium," 

 or false foot. The property of protoplasm which makes 

 this movement possible is known as contractility. It is 

 possessed in some degree by all protoplasm, but it is the 

 characteristic property of the cells and fibers of muscle. 



