AYES. 133 



The legs have a femur, a tibia and a fibula, which are connected 

 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 be- 

 low in three pulleys. 



Most commonly there are three toes before, and a thumb behind; 

 the latter being sometimes deficient. In the Martins 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 external toe, which has five. 



Birds are generally covered with feathers, the kind of tegument 

 best adapted for defending them from the rapid variations of tempe- 

 rature 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 dilatation of the chest. To each rib is an- 

 nexed 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 vicinity; a vascular 

 and plaited membrane, which stretches from the bottom of the globe 

 to the edge of the crystalline, probably assists in effecting this, by 

 displacing that lens. The anterior surface of the ball is also 

 strengthened by a circle of bony pieces, and besides the two ordi- 

 nary eye-lids, there is always a third one placed at the internal angle, 

 which, by a remarkable muscular apparatus, can be drawn over the 

 eye like a curtain. 



The breadth of the osseous openings of the nostrils determines 

 the strength of the beak; and the cartilages, membranes, feathers 

 and other teguments which narrow down those apertures, influence 

 the power of smell, and the nature of the food. 



There is but little muscular substance in the tongue, which is sup- 

 ported by a bone articulated with the hyoid; in most Birds this or- 

 gan is not very delicate. 



The feathers, as well as the quills, which only differ in size, are 

 composed of a stem, hollow at base, and of laminae, which are them- 

 selves furnished with smaller ones; their tissue, lustre, strength, and 



