218 AVES. 



are enabled to sleep in security, while perched on one foot. 

 The ischia, and particularly the ossa pubis are lengthened out 

 behind, and the interval between them is widened, in order 

 to allow the necessary space for the development of the ovum. 



Tlie neck and the beak are elongated to reach the ground, 

 but the former has the requisite flexibility for bending back- 

 wards when at rest, consequently, it has many vertebrae. 

 The trunk, on the contrary, which serves as a point d'appui 

 to the wings, has but little mobility ; the sternum, particularly, 

 to which are attached the muscles which lower the wings in 

 flight, is of great extent, and has its surface still more enlarged 

 by a salient process in its middle. It is originally composed 

 of five pieces: a middle one, of which this salient lamina 

 makes a part ; two triangular, anterior, lateral ones, for the 

 articulations of the ribs, and two posterior, which are lateral 

 and bifurcated, to increase its surface. The greater or less 

 degree of the ossification of the notches of these last, and the 

 extent of the interval they leave between them and the princi- 

 pal bone, denote a relative strength of wing and power of 

 flight. The diurnal Birds of prey, the Swallows and the 

 Humming-birds, lose, as they grow old, all traces of these- 

 unossified spaces. 



The fourchette produced by the junction of the two clavi- 

 cles, and the two powerful stretchers formed by the coracoid 

 apophyses, keep the shoulders apart, notwithstanding the 

 efforts requisite for flight, that act in an opposite direction ; the 

 greater the power of flight, the more open and strong is the 

 fourchette. The wing, supported by the humerus, fore-arm 

 and hand, the latter of which is elongated, and has one finger 

 and vestiges of two others, is furnished throughout its length 

 with a range of elastic quills, which greatly extends the sur- 

 face that resists the air. Those which belong to the hand are 

 termed primaries^ and there are always ten ; those attached to 

 the fore-arm are called secondaries, but their number varies ; 

 weaker feathers appended to the humerus are called scapulars ; 

 the bone, which is analogous to the thumb, is also furnished 



