OCR Text |
Show 218 AVES. are enabled to sleep in security, while ~erched on one foot. The ischia, and particularly the ossa pub!s a~e length~ned out behind, and the interval between them IS Widened, m order to allow the necessary space for the development of the ovum. The neck and the beak are elongated to reach the ground, but the former has the requisite flexibility for bending backwards when at rest,-consequently, it has many vertebrre. The trunk, on the contrary, which serves as a point d'appui to the wings, has but little mobility; the sternum, particularly, to which arc attached the muscles which lower the wings in flight, is of great extent, and has its sur:ace ~t~ll more enlarged by a salient process in its middle. It ~s o.ri~mall~ compo~ed of five pieces: a middle one, of which this sahent lamma 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, an~ t~e extent of the interval they leave between them and the prmCI· 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 cia~· cles and the two powerful stretchers formed by the coracoid apo~hyses, keep the shoulders apart, not~ith~tan~ing 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 attache~ t~ the fore-arm are called secondaries, but their number vanes, weaker feathers appended to the humerus are called scapulars; the bone, which is analogous to the thumb, is also furnished .AVES. 219 with what are termed bastard quills. Along the base of th quills is a range of feathers named coverts. e !he bony tail is very short; but has a range of large quills, wh1ch, when spread ?ut, ass1st in supporting the bird; they are generally twelve m number, sometimes fourteen and · th e Ga I Im. acere e1. ghteen. ' m The le~s have a femur, a tibia and a fibula, which are connected With the fem~r by_ an articulation with a spring, which keeps up the extensiOn Without any effort on the part of the ~uscles. The ~rsu~ and metatarsus are represented by one smgle bone, termmatmg below in three pullies. 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. T~e number of artic~lations increases in each toe, commencing With the thumb, which has two, and ending with the external toe, which has five. Birds are generally covered with feathers, a kind of tegu~ ent best adapted for defending them from the rapid varia~ wns ~f tem~~rature to which their movements expose them. fhe air cavities which occupy the interior of their body, and even supersede the marrow in the bones, increase their specific.lig~tness.' The. sternal, as well as the vertebral portion oft~e ribs Is ossified, m order to give more power to the dilatatiOn of the chest. To each rib is annexed a small bone which soon becomes soldered to it, and is directed oblique}; towards the next one, thereby giving additional solidity to the thorax. The eye is so constructed, in Birds, as to distinguish, with ~qual facility, objects at a distance, or in its immediate vicinIty; a vascular and plaited membrane, which stretches from the bo~to~ of th.e globe to the edge of the crystalline, probab. Iy assists m effecting this, by displacing that lens. The anter~ or surface of the ball is also strengthened by a circle of bony plec.es, and besides the two ordinary eye-lids, there is always a thud one placed at the internal angle, which, by a remark· |