70 THE BIOLOGY OF BIRDS 



the breastbone in respiration, the muscles of the gizzard if 

 there is one, the two muscles running from the windpipe 

 to the breastbone (sterno-tracheals), the muscles of the 

 song-box, the muscles moving the eye and pulling down 

 the third eyelid, the muscles of the heart, and so on, we see 

 that the bird is a very muscular animal. But expert 

 myologists tell us that the basis of the bird's muscularity is 

 already laid down in the reptile. 



Many people are familiar with the extremely hard 

 tendons of some birds. In eating the leg muscles of game- 

 birds the so-called " ossified " tendons which bind them to 

 the bones are obtrusive. The hardness has been in- 

 vestigated by E. Retterer and A. Lelievre (191 1), who find 

 that only a delicate zone has the characters of true bony 

 tissue. The main mass of the hard tendon is due to 

 hypertrophied tendinous tissue ; the cells become very 

 vesicular and the fibres are calcified. 



