ADAPTATIONS OF THE SKELETON 



47 



specialised structure has been evolved suited for over- 

 coming particular difficulties in the conditions of life. 

 Whatever interpretative theory of evolution is accepted, 

 the general fact seems clear that adaptations are the results 

 of long processes of gradual improvement in efficiency. 



§ I. Lightness and Tendency to Fusion 



Lightly Built. — The skeleton of the bird is lightly 

 built, affording a relatively large surface for the insertion 

 of muscles without the corre- 

 sponding increase of weight 

 that would be involved if the 

 bones were as substantial as 

 those of a reptile or a mam- 

 mal. The skeleton is buih 

 on the hollow girder prin- 

 ciple. The long bones have 

 large medullary cavities, and 

 many bones of many birds 

 contain prolongations of air- 

 sacs from the lungs. The 

 marrow often disappears en- 

 tirely at an early age ; and 

 while the bone is still a-mak- 

 ing an air-sac enters its cavity. 

 In some birds, like the alba- 

 tross, practically every bone fig. 5. -Thigh-bone or femur of 

 is " pneumatic " except the 

 scapula and the hyoid. In 

 the case of skull-bones the 

 air-cavities are in communi- 

 cation with the nasal chamber 

 and auditory passage, not with 



the lungs. But although the bones are not massive, they are 

 not in any way weak. There is a hard dense cortex, which 

 does not attain great thickness, and below this comes a 

 framework of spongy texture, especially marked in Running 



an 



ostrich, cut open to left. From 

 a specimen. H., head of femur, 

 which works in the hip-joint or 

 acetabulum ; pn.f., pneumatic for- 

 amen, where air-sac enters ; pn.c, 

 pneumatic cavity ; CO., dense cor- 

 tex ; SP., spongy texture. 



