168 ELEMENTS OF ORNITHOLOGY. 
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 back-bone, or spine, which supports, 
at its anterior end, the skull. From either side of part of the 
back-bone, the vids proceed outwards, and are attached at their 
other ends to a breast-bone 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-skeleton. 
The axial skeleton—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—z.¢., with the “ spinal” and 
* thoracic” parts of the skeleton. 
* So called because it answers to the parts which form the skeleton of 
our chest or ‘“ thorax.” 
