202 From Matter to Man. 



Birds. — In the mechanism of a fast-flying bird, its 

 shape is skilfully adapted to cleaving the air. Its 

 bones are, for lightness, hollow cylinders, and so 

 constructed for strength that weight for weight they 

 are six times as strong as cast iron. The hollow 

 spaces connect with the lungs, and can be filled with 

 air or emptied as occasion requires. As in the insect, 

 abundant means also exist for purifying and aerating 

 the blood. An engine burns fuel according to the 

 amount of work done ; so does an organism. Some 

 birds (the swallow and sparrow) are model high- 

 pressure engines, consuming as much as their own 

 weight of food per day. There is a vast difference, 

 however, between the natural and artificial machine 

 in the quantity of fuel expended and the resultant 

 energy obtained. Thus the human body utilises 

 every fraction of its heat, while the locomotive wastes 

 90 per cent, of its fuel. 



All the motions of animals are effected by the 

 shortening or contraction of their muscles. The 

 great muscle which moves the wing of an albatross 

 contracts at each stroke with a force of about 30 lbs. 

 The rapidity of the wing-strokes of birds is also 

 amazing. Thus the lazy-flying heron executes 150 

 strokes per minute ; the wild duck 540 ; and the 

 sparrow, 780. Again, the hawk for lung distances 

 attains a speed of 150 miles an hour The energy 

 which accomplishes all this activity is said to result 

 from electricity evolved by chemical action ; every 

 nerve and muscle in their movements receiving and 



