22 



THE GROWTH OF ANIMALS 



Organs of animals. — The body of an ox is made up of 

 many different parts, or organs, and each organ has a certain 

 function, or work, to perform. For example, the lungs per- 

 form the function of purifying the blood, the legs are for 

 locomotion, the ears for hearing, and the eyes for seeing. 

 We may define an organ, then, as a part of an animal that 

 has some definite function to perform. 



The relation of function to structure. — The leg of a duck 

 performs the double function of walking and swimming. 



Fig. 8. — Foot of duck, blue jay, mole, and seal, respectively, showing 



adaptation to function. 



It is accordingly provided with joints, muscles, toes, and a 

 web between the toes. The feet of a blue jay are fitted for 

 perching. The hmbs of a seal are fashioned for swimming 

 and the feet of a mole for diggmg (Fig. 8). The teeth of a 

 dog are constructed for tearing flesh, but the teeth of an 

 elephant are suited to grinding vegetable material. From 

 these examples, it will be seen that the structure of an or- 

 gan is suited to the work, or function, it has to perform. 

 The higher we go in the scale of animal life, the more com- 

 plex becomes the function and structure of organs. This 

 is shown by the fact that a bird's wings are much more 



