THE SKELETON. 35 



forms one great air-cell, with thin bony parietes ; and in this bird, in 

 the Swifts, and the Humming-birds, every bone of the skeleton, down 

 to the phalanges of the claws, is pneumatic. 



The extent to which the skeleton is permeated by air, varies in 

 different birds, in relation chiefly to their different kinds and powers 

 of flight. The opposite extreme to the Swift is met with in the 

 terrestrial Apteryx and aquatic Penguin, in wliich not any bone of the 

 skeleton receives air. 



In the mammalian class the air-cells of bone are confined to the 

 head, and are filled from the nasal or tympanic cavities, never from 

 the lungs. The frontal, sphenoidal, and maxillary sinuses, and the 

 mastoid cells, are examples of pneumatic bones in the human subject. 

 The frontal sinuses extend backwards over the calvarium in most 

 Ruminants, and penetrate the cores of the horns in oxen, sheep, and 

 a few antilopes. 



The whole diploe of the upper, back, and side Avails of the cranium 

 was inflated, as it were, with air in the great extinct Sloths ; the 

 outer table was raised considerably above the vitreous, and the brain 

 thus seemingly defended by a double skull; the advantage of which 

 modification to these leaf-devouring animals, in the event of blows 

 from the falling trees which they uprooted, is well displayed in the 

 healed fractures of the skull of the Mylodon, in the museum of the 

 College (xii. p. 157.). The outer table of the entire epicranium is 

 similarly raised above the inner one by intervening large air-cells, 

 and their sinuous septa, in the Giraffe ; the short horns are solid, but 

 are sustained by the vaulted roof of the skull ; and, as the animal 

 can deal heavy blows with these simple weapons, the concussion is 

 diminished by the interposition of these air-chambers between tlie 

 outer table and the immediate covering of the brain. 



The most remarkable development of air-cells in the mammalian 

 class is, however, presented by the Elephant ; the intellectual phy- 

 siognomy of this great Pachyderm being caused, as in the Owl, not 

 by actual capacity of the brain-case, but by the enormous extent of 

 the pneumatic cellular diploe between the two tables of the skull. 



In each of these modifications the vacuities of the osseous tissue, 

 whether mere cancelli as in the Tortoise, or small medullary cavities 

 as in the Crocodile, or larger medullary cavities as in Mammals, or 

 pneumatic cavities and sinuses, are the result of secondary changes 

 by absorption, and not of the primitive constitution of the bones. 

 These are in all air-breathing animals solid at their first commence- 

 ment, and the vacuities are formed by the removal of osseous matter 

 previously formed, whilst fresh bone is added to the exterior surface. 



