THE LIVING WORLD 187 



hollow fibres, pith, leaf-substance, &c. Now these 

 various substances are called tissues, and each, when 

 examined by the microscope, is found to be composed of 

 very small structures, which are for the most part known 

 as " cells." The study of tissues and their minute 

 structure is called histology. 



Anatomy shows us that living creatures are composed 

 of various parts of organs, each made up of certain 

 tissues. Thus the cat, like man, is provided with a 

 mouth to receive food, teeth to divide and crush it, a 

 stomach to digest it, an alimentary canal and liver, a 

 heart and blood-vessels, a brain and nerves, &c., each 

 being composed of the various tissues which enter into 

 its composition. An oak also has its stem, containing 

 many tubular vessels, roots which spread into the soil, 

 and leaves which expand and expose themselves to the 

 sun's rays. All these different parts or organs concur in 

 sustaining the life of the creature which possesses them, 

 and therefore the life of its various other organs. 



In this way each organ is reciprocally "end" and 

 "means"; on which account living creatures are com- 

 monly spoken of as organisms. Organs are also united 

 together into groups which are called il systems" Thus 

 the mouth, stomach, intestine, liver, &c., constitute what 

 is called the alimentary system; the heart and vessels 

 form the vascular or circulating system, and the brain 

 and nerves the nervous system. 



Now all these various component parts of living crea- 

 tures have their respective activities or functions, which 

 minister to, and are subsumed in, the life of the creature 

 itself. The study of these various functions or activities 

 is termed physiology. 



Each." cell " has its own activity, as has each " tissue " 

 (formed of cells of one kind) and each " organ " (formed of 



