THE CELL 



31 



Composition of organs. — If we turn now to an exami- 

 nation of the organs, we find them made up of masses of 

 material which differ both in appearance and action. 

 For example, the nose consists of a hard framework of 

 bone, an outer covering of skin, and underlying portions of 

 red flesh, blood vessels, and nerves. Other organs, when 

 similarly examined, are found to contain also these flesh, 

 blood-vessel, and nerve la}^ers, but in different positions. 

 These substances of which the organs are built are called 

 tissues, and the simplest structural units to which we can 

 reduce the body with the aid of the knife are these tissues. 

 Tissues then are the layers of matter of similar appear- 

 ance which combine to form the organs. 



If now we call the microscope to our aid, tissue, in turn, 

 is seen to be made up of tiny bodies of definite structure 

 and shape called cells. 



The cell. — If we scrape the inside of the cheek with a 



Fig. 8 — Plant and animal cells ; A, plant cell (spirogyra); 5, animal cells scraped 

 from the inside of the cheek; C.TF., cell wall; C, colored bodies; P, proto- 

 plasm ; S.B., spiral band ; N, nucleus ; C.S., cell sap. 



clean knife and mount the scrapings in a drop of water, for 

 examination under the microscope (see Ex. XXII.), these 



