22 



ELEMENTARY BIOLOGY 



Fig. 4. Various kinds of animal cells 



/, flat epithelial cells, like those lining the cavity 

 of the abdomen in man and other animals ; 2, co- 

 lumnar epithelial cells, like those lining the air pas- 

 sages, with hairlike projections of protoplasm, called 

 cilia ; _j, muscle cells, unstriped, like those in the 

 walls of the intestine and of blood vessels; 4, 

 shapeless cells of naked protoplasm, like those of 

 Ameba or of white blood corpuscles ; j, cells con- 

 taining fat globules, like those in adipose tissue ; 

 6, bone cells surrounded by hard deposits of limy 

 material ; 7, a nerve cell, or ncuivn {a, the cell 

 body with its branching outgrowths, or dendrites; 

 b, the longest outgrowth, the axmi, ending in c, the 

 terminal branches) 



35. Cells. It has been 

 known for a long time 

 that the body of every 

 plant and eveiy animal 

 is made up of a large 

 number of tiny lumps 

 of protoplasm, each of 

 which is shut off from 

 its neighbors by a more 

 or less definite mem- 

 brane, or wall. A single 

 bit of protoplasm with its 

 wall is known as a cell. 

 This name suggested 

 itself to those who first 

 studied the structure 

 under the microscope, 

 because of its resem- 

 blance to the cells of a 

 honeycomb. When we 

 look at a living organ- 

 ism, we do not see 

 the protoplasm ; we see 

 the walls of thousands 

 of these cells. In the 

 larger plants and animals 

 the outer layers of cells 

 are usually quite dead- — 

 that is, the protoplasm 

 is no longer present, 

 only the dead wall re- 

 maining. This is true 

 of our own skin, of the 

 bark of trees, and of 

 the hide of the horse. 



