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; 3, muscle c , 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 neuron (a, the cell 

 body with its branching outgrowths, or dendritcs; 

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

 terminal branches) 



35. Cells. It has been 

 known for a long time 

 that the body of every 

 plant and every 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. 



