SKELETON AND MUSCLES. 145 



The number of joints in the thumb of birds is two, in 

 the next toe three, in the next four, and in the outer one 

 five. The bones of birds are hollow, as already stated ; 

 hence very light in comparison with their size and 

 strength. They are more laminated, and less fibrous, 

 than the bones of any other vertebrates. 



The bones of the Penguin a bird which does not fly are solid and 

 heavy ; and those of the Apteryx a wingless bird of New Zealand 

 are nearly solid, having only a small medullary cavity in the femur.* 



The muscles of birds are relatively large and powerful. 

 And here it may be stated as an interesting fact, that 



FIG. 161. 



V,'W!& 



a 



Position of the leg of a Bird ill perching. 



The weight of the bird, by bending the joint 6 and the joint c, puts the muscle a, 

 6, c, d upon the stretch, and thus flexes the toes. 



* The bones of all birds, and of all other vertebrates, are at one stage of 

 their formation essentially solid ; that is, they are not hollow, and they 

 have no cavities of any sort. The hollowness and the cavities are produced 

 by the removal by absorption of bony tissue previously formed; and 

 thus is the bone adapted for the function it is to perform. " The thinnest- 

 walled and widest air-bone of the bird of flight was first solid, next a mar- 

 row-bone, and finally became the case of an air-cell." OWEN. 



