228 ZOOLOGY. 



birds have the largest bones in proportion to their 

 weight, and their bones are more hollow than those 

 of animals that do not fly. Birds can also, by means 

 of air vessels, blow out the hollow parts of their 

 bodies, when they wish to make their descent slower, 

 rise more swiftly, or float in the air. 



The muscles that move the wings of a bird down- 

 wards, in many instances constitute not less than the 

 sixth part of the weight of the whole body ; whereas 

 those of a man are not in proportion one hundredth 

 part so large. 



Eyes of Birds. 



Birds can suddenly change the form of their eyes, 

 by means of a set of hard scales, drawn by fibres, 

 placed on their outer coat, round the part where the 

 light enters; so that by acting on these, it can 

 squeeze the natural magnifier of the eye into a round 

 shape while following an insect in the air, and relax 

 them in order to flatten them, to see distant objects, 

 such as twigs and branches. This power of relaxing 

 and contracting the eye is possessed in a more re- 

 markable degree by birds of prey. A singular pro- 

 vision is also made to enable a bird to wipe its eye 

 clean a kind of thrid eye-lid. 



The Ostrich. 



The ostrich lays and hatches her eggs in the sand, 

 her form being ill adapted for that process, while the 

 sand furnishes a natural oven, and the sun supplies 

 the stead of animal warmth. 



The Goat. 



The goat eats hemlock (which destroys sheep) 

 with impunity, and readily devours the smaller kinds 

 of serpents. The Tibet goats eat horse-chestnuts, of 

 which they are particularly fond, weeds, and most 

 trash that other animals, even the pig, reject. 



