12 SKELETON OF BIRDS. 



while the thoracic or anterior extremities never serve them for 

 walking, nor for prehension, nor for touch ; but they form a sort 

 of broad oars, named wings, which, by striking the air, sustain 

 and cause the animal to move in it. 



5. The Skeleton, {Plate I, Jig. 1-) which determines the 

 general form of the body, and which is. at the same time, one 

 of the most important parts of the apparatus of motion, is com- 

 posed of nearly the same bones as that of the mammalia ; but 

 their form and disposition vary. 



6. The head is small, the bones of the cranium are soldered 

 together at an early period of life, and the face is formed almost 

 entirely by the jaws which are very much elongated and consti- 

 tute a beak. The superior mandible or jaw is articulated with the 

 cranium, in such a manner as to allow some mobility, and the 

 lower mandible, in place of being articulated directly with the 

 cranium, as is the case in mammalia, is suspended from a moveable 

 bone, called the square or tympanic bone, which is articulated with 

 the petrous bone ; [this mode of articulation of the lower jaw is 

 met with also in other oviparous vertebrate animals, that is, in 

 fishes and reptiles.] These mandibles are composed of many 

 pieces, and are enveloped in a horny substance which takes the 

 place of teeth. 



7. The articulation of the head with the vertebral column rs 

 much more moveable than it is in mammals, and is effected 

 through the means of a single rounded eminence, (called con- 

 dyle,) while in the mammalia there are always two of these 

 condyles. This arrangement enables the bird to direct his face 

 entirely and completely backwards. 



8. The neck of birds is also very moveable ; and as these ani- 

 mals generally take their food from the ground with their beak, 

 the length of this part of their body is necessarily in proportion 

 to the height at which they are placed on their legs. This is in- 



5. In \Ahat respect does the skeleton of birds differ from that of mammals ? 



6. What is remarked of the head of birds ? What forms the face? How 

 does the articulation of the upper jaw with the cranium differ in birds, from 

 the same articulation in the mammalia ? What is the peculiarity of the 

 articulation of the lower jaw in birds ? With what bone does tlie square- 

 bone articulate ? Is this mode of articulation of the lower jaw peculiar to 

 birds ? How are these mandibles composed ? 



7. What is the peculiarity of the articulation of the head (of birds) with 

 the vertebral column ? . What is the advantage resulting from this arrange- 

 ment ? 



8. Upon what circumstances does the length of the neck seem to depend? 

 What is the most common number of cervical vertebrae in birds ? How 

 many cervical vertebra has the Swan ? How many has the Sparrow ? Are 

 the bones of the neck very moveable on each other ? 



