GENERAL ORNITHOLOGY 



in the lungs ; such sacs, sometimes of great extent, are also found 

 in many places in the interior of the body, beneath the skin, etc.; 

 sometimes the whole subcutaneous tissue is pneumatic. The extent 

 to which the skeleton is aerated is very variable. In many birds 

 only the skull, in a few the entire skeleton, is in such condition ; 

 ordinarily the greater part of the skull, and the lesser part of the 

 trunk and limbs, is pneumatised. The passage of air in some cases 

 is so free, as into the arm-bone, for example, that a bird with the 

 windpipe stopped can breathe for an indefinite period through a 



Fig. 54. — Ideal plan of the double-ringed 

 body of a vertebrate. A'', neural canal ; if, 

 liKmal canal ; the body separating them is 

 the centrum of any vertebra, bearing e, an 

 epapojjhysis, and i/, a hypapophysis ; m, ■», 

 neurapophyses ; d, d, diapophyses ; ?w, 

 bifid neural spine ; pi, pi, pleurapophyses ; 

 h h, hitmapophyses ; hs, bifid htemal .spine. 

 Drawn by Dr. R. W. Shufeldt, U.S.A., after 

 Owen. 



Fig. 55. — Actual section of the body in the 

 thoracic region of a bird. JV, neural canal ; H, 

 hsemal canal ; c, centrum of a dorsal vertebra ; 

 hy, hypapophysis ; d, diapophysis ; ^, zygapo- 

 physis ; ns, neural spine ; r, pleurapophysis, or 

 vertebral part of a free rib, bearing v, uncinate 

 process or epipleura ; cr, heemapophysis or 

 sternal part of the same ; st, section of sternum 

 or breast-bone (hremal spine). Designed by Dr. 

 R. W. Shufeldt, U.S.A. 



hole in the humerus. Pneumaticity is not directly nor necessarily 

 related to power of flight ; some birds which do not fly at all are 

 more pneumatic than some of the most buoyant. (On the general 

 pneumaticity of the body see beyond, under head of the Respiratory 

 System.) 



The Axial Skeleton (Figs. 54, 55, 56) of a bird or any vertebrated 

 animal, that is, one having a back-bone, exhibits in cross-section two 

 rings or hoops, one above and the other below a central point, like 

 the upper and lower loops of a figure 8. The upper ring is the 

 neural arch (Gr. vevpov, neuron, a nerve), so called because such a 



