82 TYPES OF ANIMAL LIFE 



immediately when any injury has paralysed its muscular 

 power, as every sportsman knows. 



That such feats of flight should be performed by a 

 creature so solidly built needs a very careful and a very 

 peculiar arrangement of the parts and organs which 

 compose its body. The various organs are indeed so 

 packjd and arranged as to make the centre of the body's 

 gravity fall just where it can be best sustained, and 

 they are so constructed as to produce, in combination, 

 the greatest strength and warmth with the least possible 

 weight. But no dissection is needed in order that we 

 may see how admirable a bird's organisation is. Its 

 veiy external clothing shows us this, foi- it is made 

 up of feathers, and a feather is a very marvel of 

 skilful construction. It is a structure at once so light 

 and so strong, so admirably adapted to retain the heat of 

 the body that it is hardly possible to imagine anything 

 of the kind more perfect. 



A bird's bones must be strong for the work they have 

 to perform, and to be strong they must possess a certain 

 solidity, and therefore w^eight, but this weight is com- 

 monly reduced to the minimum necessary for the needful 

 strength. This reduction is effected partly by a .skilful 

 arrangement of the solid parts themselves, but it is 

 further effected by the bones containing not a mass of 

 marrow, but warm (and therefore light) air. In some 

 birds even each bone of the toes is thus furnished, while 

 air passes into the bones of the skeleton freely by means 

 of passages, which extend to them from the lungs. But 

 in order that a bird may be able to fly, great power 

 is no less indispensable than is lightness of structure, 

 and to move the wings as they need to be moved for 

 flight, very large muscles are necessary. Large muscles 

 are necessary to raise as well as to depress the wings 



