MUSCULAR SYSTEM. 165 



only the long narrow tendons ; hence the long slender legs 

 and the lightness of the arms in this class. As birds have 

 nearly all the same general form and movements, there is 

 remarkable uniformity in the muscular system throughout 

 this class. Their trunk being almost as fixed as in the che- 

 lonia, the muscles destined to move that part are here also 

 very imperfectly developed, but its fixed condition affords 

 the most advantageous attachment for the powerful muscles 

 of the extremities, which have generally a very high origin. 

 The arms and hands being entirely appropriated to flight, 

 and the legs and feet being constructed chiefly for support, 

 their long flexible neck and highly moveable head supply the 

 place of anterior extremities for all ordinary purposes of 

 prehension, and the cervical region is therefore strong, move- 

 able, and muscular, like the trunk of a serpent. The face of 

 the birds, from the imrnoveable horny sheaths which cover 

 the exterior of the jaws, is like that of the crocodilian and 

 chelonian reptiles, destitute of many muscles destined to 

 move the fleshy lips in other classes. The temporal, (Fig. 

 76. ,) the masseter, and the pterygoid muscles are powerful 

 in birds, and correspond with the great length and the mo- 

 bility of their lower jaw ; they are stronger in the rapacious 

 birds, from the resistance they have to overcome, and they 

 are most unequally developed on the two sides of the head 

 in the crossbills, which habitually draw the lower jaw to one 

 side of the head. The large eyes in this class are moved by 

 four recti muscles, as in mammalia, and have their middle 

 portion surrounded, compressed, and rotated by two obliqui. 

 The orbicularis palpebrarum, expanded over their large orbit, 

 acts most on the lower eye-lid, as in inferior classes ; the 

 lower eye-lid has also its small depressor, as the upper has 

 its levator muscle, and the membrana nictitaus, with its long 

 inferior tendon passing behind and over the ball, is moved 

 with great velocity by its pyramidalis and quadratus muscles. 

 The fixed condition of the dorsal vertebrae affords a more 

 solid attachment to the muscles of the long neck, and in 

 rapaceous birds, which frequently carry their prey suspended 

 from their jaws, to a distance through the air, the neck is 

 comparatively short and strong, and the muscles of great 

 power which move these vertebrae, as seen in the annexed 

 figure from Carus, representing the muscles of the falco 



