AVES. 219 



with what are termed bastard quills. Along the base of the 

 quills is a range of feathers named coverts. 



The bony tail is very short, but has a range of large quills, 

 which,, when spread out, assist in supporting the bird; they 

 are generally twelve in number, sometimes fourteen, and in 

 the Gallinacese eighteen. 



The legs have a femur, a tibia and a fibula, which are con- 

 nected with the femur by an articulation with a spring, which 

 keeps up the extension without any effort on the part of the 

 muscles. The tarsus and metatarsus are represented by one 

 single bone, terminating below in three puUies. 



Most commonly there are three toes before, and a thumb 

 behind ; the latter being sometimes deficient. In the Mar- 

 tins it is directed forwards. In the Climbers, on the contrary, 

 the external toe and the thumb are directed backwards. 

 The number of articulations increases in each toe, commencing 

 with the thumb, which has two, and ending with the exter- 

 nal toe, which has five. 



Birds are generally covered with feathers, a kind of tegu- 

 ment best adapted for defending them from the rapid varia- 

 tions of temperature to which their movements expose them. 

 The air cavities which occupy the interior of their body, and 

 even supersede the marrow in the bones, increase their 

 specific lightness. The sternal, as well as the vertebral portion 

 of the ribs is ossified, in order to give more power to the di- 

 latation of the chest. To each rib is annexed a small bone, 

 which soon becomes soldered to it, and is directed obliquely 

 towards the next one, thereby giving additional solidity to the 

 thorax. 



The eye is so constructed, in Birds, as to distinguish, with 

 equal facility, objects at a distance, or in its immediate vicin-. 

 ity ; a vascular and plaited membrane, which stretches from 

 the bottom of the globe to the edge of the crystalline, proba- 

 bly assists in effecting this, by displacing that lens. The ante- 

 rior surface of the ball is also strengthened by a circle of bony 

 pieces, and besides the two ordinary eye-lids, there is always 

 a third one placed at the internal angle, which, by a remark- 



