106 BLEMBNTS Or OENITHOLOGT. 



be known to us by their fossil remains alone. The skeleton 

 also bears a close relation to the general external form, and its 

 structure is a guide to the actions and therefore to the modes 

 of life of the living organism of which it once formed the 

 basis and support. 



Various parts of the skeleton are capable of being moved one 

 upon the other, and to facilitate these motions the contiguous 

 surfaces of such moveable bones are so shaped as to form what 

 are known as " joints." 



The skeleton consists of the hack-hone, or spine, which supports, 

 at its anterior end, the sTcull, From either side of part of the 

 back-bone, the ribs proceed outwards, and are attached at their 

 other ends to a hreast-hone or sternum. All this constitutes 

 the axial part of the endo-skeleton. The back-bone con* 

 stitutes the spinal part of the axial skeleton, while the skull 

 forms its cranial portion. The rest of these parts — the breast- 

 bone and ribs — together make up the thoracic * part of the 

 skeletal axis. Two pairs of bony girdles are attached to the 

 axial skeleton, and each such girdle supports the bones of one 

 of the two pairs of limbs. 



The anterior, or pectoral, limb-girdle is attached to the front 

 of the sternum, and to it the wing-bones are affixed. 



The posterior, or pelvic, limb-girdle is much more solidly 

 connected with the more posterior part of the back-bone than 

 is the pectoral girdle with the breast. It forms the sockets 

 into which the upper ends of the thigh-bones fit. 



These two bony limb-girdles, with the bones of the wings 

 and of the lower limbs, together constitute the appendicular 

 portion of the endo-skekton. 



The axial slceleton. — As we began by considering the exo- 

 skeleton of the head, trunk, and tail before that of the limbs, 

 so we will here begin by first studying the bony framework, 

 or endo-skeleton, of the head, trunk, and tail. 



It will, however, be more convenient to defer considering the 

 skeleton of the head, till we have made acquaintance with the 

 bony parts of the trunk and tail — i. e,, with the " spinal " and 

 " thoracic" parts of the skeleton. 



* So called beeause it answers to the parts which form the skeleton of 

 our chest or " thorax." 



