THE ANATOMY OF BIRDS. — OSTEOLOGY. 135 
The skeleton of birds is noted for the number and extent of its anchyloses, a great ten- 
dency to codssification 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 pnewmaticity (Gr. mvevparixds, pneumatikos, 
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 aérated 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 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 
Fie. 54. — Ideal plan of the double-ringed body of a 
vertebrate. W,neuralcanal; H, hemal canal; the body 
Fie. 55. — Actual section of the body in the thoracic 
region of a bird. N, neural canal; H, hemal canal; c, 
separating them is the centrum of any vertebra, bear- 
ing e, an epapophysis, and y, a hypapophysis; n,n, neu- 
rapophyses; d, d, diapophyses; ns, bifid neural spine; 
pl, pl, pleurapophyses; h, kh, hemapophyses; hs, bifid 
centrum of a dorsal vertebra; hy, hypapophysis; d, 
diapophysis ; z, zygapophysis; ms, neural spine; 7, 
pleurapophysis, or vertebral part of a free rib, bearing 
u, uncinate process or epipleura; cr, haemapophysis 
hemal spine. Drawn by Dr. R. W. Shufeldt, U.S. A., 
after Owen. 
or sternal part of the same; st, section of sternum or 
breast-bone (hamal 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.) 
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. vedpov, neuron, a nerve), socalled because such a cylinder encloses a section 
of the cerebro-spinal axis, or principal nervous system of a vertebrate (brain and spinal cord, 
