THE ANATOMY OF BIRDS. — OSTEOLOGY. 



136 



Tlie skeleton of birds is noted for the number and extent of its anchyloses, a great ten- 

 denc}' to coossification and condensation of bone-tissue resulting from the energy of the vital 

 activities in this hot-blooded, quick-breathing class of creatures. Birds' bones are remarkably 

 hard and compact. When growing, they are solid and marrowy, but in after life more or fewer of 

 them become hollow and are filled with air. This pneumaticity (Gr. irvevixariitos, imeumatikos, 

 windy) is highly characteristic of the avian skeleton. Air penetrates the skull-bones from the 

 nose and ear-passages, and may permeate all of them. It gains access to the bones of the 

 trunk and limbs by means of air-tubes and air-sacs which connect with the air-passages 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, 

 iu 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 pneumatized. 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 



Fig. 55. — Actual section of the body In the thoracic 

 region of a bird. N, neural canal ; E, hsemal canal ; c, 

 centrum of a dorsal vertebra; hy, hypapophysls ; d, 

 diapophysis ; z, zygapophysls ; ns, neural spine; r, 

 pleurapopbysis, or vertebral part of a free rib, bearing 

 M, uncinate process or epipleura; cr, hsem apophysis 

 or sternal part of the same; st, section of sternum or 

 breast-bone (hajmal spine). Designed by Dr. R. W. 

 Shufeldt, U. S. A. 



for an indefinite period through 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.) 



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

 . vertebrate. X, neural canal ; H, hoemal canal ; the body 

 separating them Is the centrum of any vertebra, bear- 

 ing e, an epapophysis, and y, a hypapophysls; n, n, neu- 

 rapophyses; d, d, dlapophyses; ns, bifid neural spine; 

 pi, p/, pleurapopliyses; /;,/*, hsemapophyses; /is, bifid 

 haemal spine. Drawn by Dr. R. W. Shufeldt, U. S. A., 

 after Owen. 



The Axial Skeleton (figs. 54, 55, 56) of a bird or any vertebrated anhnal, 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 cylinder encloses a section 

 of the cerebro-spinal axis, or principal nervous system of a vertebrate (brain and spinal conl, 



