298 THE ANATOMY OF VEETEBEATED ANIMALS. 



the PaiTots, Toucans, Cuckoos, Woodpeckei's, and other so- 

 called " scansorial " birds, the outer toe is permanently re- 

 versed. Under these circumstances the distal end of the 

 outer metatarsal 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 backwards. Lastly, in the Swifts, the Dyspormncrrph ce 

 and the Spheniscomorplim, the hallux is directed more or less 

 forwards, so that all four toes are turned to the front. 



As a general rule, the osseous tissue of birds is remark- 

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

 and fiUed with vasciilar medulla ; but, after birth, more or 

 fewer of the bones are always excavated by prolongations 

 of cavities containing air, which lie in their neighbourhood. 

 Such air cavities are always found in the skull, in connec- 

 tion 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, Pen- 

 guins, 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 off prolongations into the 

 bones of the rest of the skeleton, and thus the whole skeleton 

 in some cases (as in the Horubills) 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 GruU. . 



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

 neath the integument. 



