82 AVES. 



close behind the os quadratum, passes internally thence into the 

 pneumatic opening in the posterior part of the lower jaw. Now in 

 the Passerine birds this tube is very generally surrounded by a bony 

 sheath (siphonium Nitzch), which is absent in the remaining orders ; 

 in the larger species, such as Corvus, which are provided with a 

 muscular vocal apparatus, this sheath of bone is very large and con- 

 spicuous. Another ossicle which constantly occurs in the Rapaces, 

 Passeres, and Scansores, is called the humero-capsular, from its 

 being situated within the articulating capsule of the shoulder, 

 where it plays by means of a surface invested with cartilage upon 

 the upper part of the head of the humerus. Frequently among 

 the Passeres, as in Cypselus, a kind of patella is found in the 

 tendon of the long extensor of the arm, behind the olecranon ; it 

 is even double in Aptenodytes, where two large patelliform bones 

 are met with. Two other sesamoid bones (epicarpium and hypocar- 

 pium) are situated on the carpus, which serves as a point of attach- 

 ment to the primaries of the wing. Two ossicles (ossa potato man- 

 dibularia) are occasionally situated upon the upper edge of the lower 

 jaw, as in Fulica. In the ligament which passes from the jugal 

 bone to the lower jaw, one or two ossicles are situated in many 

 birds. Some others occur also in the lower extremities, as in many 

 Mammalia. 



MUSCULAR SYSTEM. 



THE muscles of Birds exhibit in general, in conformity with the 

 whole organization of this class, fewer diversities of importance than 

 those in the Mammalia and Amphibia. The muscles of most Birds 

 are remarkable for their deep red color and the density of their tex- 

 ture. In the herbivorous birds the muscles are of a paler color, and 

 softer, and their intervening cellular tissue contains a greater quan- 

 tity of fat, so that the flesh is also more palatable as an article of 

 food. The mechanical arrangement and position of the muscles is 

 specially adapted to the performance of the function of flight ; the 

 fleshy parts of the muscles, or their bellies, are very short and thick, 

 and concentrated upon the trunk, so as not to encumber the limbs, 

 the muscles of which lie as much as possible upon the body, and 

 are extended into long tendons, which exhibit a peculiar disposition, 

 especially in the posterior extremities, to become ossified to a con- 

 siderable extent. 



The tegumentary muscles which contract the skin are very ex- 



