24 STRUCTUKE OF BONES IN DIFFERENT CLASSES. 



the "pneumatic foramen," near one end of tlie bone, com- 

 municates with its interior, and an air-ceU, or prolonga- 

 tion of the lung, is continued into and lines the cavity of 

 the bone, which is thus filled with rarefied air instead of 

 marrow. The extremities of such air-bones present a 

 light open network, slender columns shooting across in 

 different directions from wall to wall, and these little 

 columns are likewise hollow. 



The enormous beak of the hornbill, which seems at 

 first sight to constitute so grave an impediment to flight, 

 forms one enormous air-cell, with very thin bony walls; 

 and in this bird, in the swifts, and the humming-birds, 

 every bone of the skeleton, down to the last joints of the 

 toes, is permeated by hot air. The opposite extreme to 

 the above members of the feathered class is met with in 

 the terrestrial apteryx (wingless bird of New Zealand), 

 and in the aquatic penguin; in both of which, not any 

 bone of the skeleton receives air. Intermediate grada- 

 tions in the extent to which the skeleton is permeated by 

 air occur in different birds, and in relative proportion to 

 their difterent kinds and power of flight. 



In the mammalian class, the air-cells of bone are con- 

 fined to the head, and are filled from the cavities of the 

 nose or ear, not from the lungs. Such cells are called 

 "frontal sinuses," "antrum," "sphenoidal" and "ethmoidal 

 sinuses" in man. The frontal sinuses extend backwards 

 over the top of the skull in the ruminant and some other 

 quadrupeds, and penetrate the cores of the horns in oxen, 

 sheep, and a few antelopes. The most remarkable de- 

 velopment of air-cells in the mammalian class is presented 

 by the elephant; the intellectual physiognomy of this 

 huge quadruped being caused, as in the owl, not by the 



