6 
‘animal to be sustamea in the air. If we can diminish the 
weight of the osseous system, we may lessen, in a greater pro- 
portion, the weight of the muscular; for there will be less 
muscle, as well as less bone to carry. In birds the osseous 
framework is made as light as is consistent with the strength 
necessary, by reducing as far as possible the quantity of osseous 
matter, and so disposing this on mechanical principles as to offer 
the greatest possible resistance to the action of the muscles. To 
this end the bones are hollow, and the bony matter deposited in 
superficial lamelle, dense, but of no great thickness. Air is 
admitted into their cavities, to absorb the aqueous matter se- 
ereted by the endosteum, and being renewed gradually by the 
respiratory movements, carries off the moisture that would other- 
wise collect, fill the cavities of the bones, and add greatly to 
their weight. The cellular tissue of the lungs is reduced to 
a minimum, with the double effect of rendering them lighter, 
and facilitating hematosis; and, finally, voluminous air-vessels, 
of great tenuity, and containing large supplies of air for re- 
spiration, without materially increasing the weight of the body, 
conduce to make the respiratory apparatus of birds one of the 
most striking examples of the perfect adaptation of means to 
ends to be met with in the animal kingdom. 
Our great physiologist, John Hunter, believed it impossible 
that the ribs and sternum of a bird could move while the 
powerful pectoral muscles are engaged in flight. He therefore 
thought that the air-sacs of birds might be intended to act as 
reservoirs of air to be used in respiration during flight. These 
sacs, however, do not hold enough air to support the respiration 
of a bird for two minutes; for in that time, if the trachea of a 
bird be tied, it dies; yet many birds continue on the wing for 
hours together. Sappey has endeavoured to explain the difficulty 
of Hunter by pointing out that the great pectoral muscles of 
birds arise exclusively from the sternum, and not at all from the 
ribs, as they do in mammals. But this explanation only removes 
