THE FOOT IN BIRDS. 255 



be turned outward, or even backward, at will. And in the 

 Parrots, Toucans, Cuckoos, Woodpeckers, and other so-called 

 " scansorial " birds, the outer toe is permanently reversed. 

 Under these circumstances the distal end of the outer meta- 

 tarsal may be divided into two distinct articular surfaces. In 

 the Trogons, there are two toes in front and two behind, as 

 in the Parrots, but it is the second toe which is turned back- 

 ward. I.astly, in the Swifts, the Dysporomorphoe and the 

 Spheniscomorphoe, the hallux is directed more or less forward, 

 50 that all four toes are turned to the front. 



As a general rule, the osseous tissue of birds is remarkably 

 dense and hard. Before hatching, the bones are solid and 

 filled with vascular medulla ; but, after birth, more or few^er 

 of the bones are always excavated by prolongations of cavities 

 containing air, which lie in their neighborhood. Such air- 

 cavities are always found in the skull, in connection with the 

 nasal and auditory passages, and they may extend through all 

 parts of the skull, with the exception of the jugal arch. In 

 many birds, such as the Apteryx^ Penguins, Divers, Gulls, and 

 the smaller song-birds, no other bones than those of the skull 

 are pneumatic ; but, in most birds, the air-sacs of the lungs 

 send oif prolongations into the bones of the rest of the skele- 

 ton, and thus the whole skeleton in some cases (as in the 

 Hornbills) becomes pneumatic. It is proper to remark that 

 the amount of pneumaticity of the bones by no means follows 

 the development of the power of flight. In the Ostrich, for 

 example, the bones are far more extensively pneumatic than 

 in the Gull. 



In some cases, prolongations of the air-sacs extend beneath 

 the integument. 



