AVES. 



279 



stituting what is known as the " down." In the Ostriches and the 



birds allied to them, all the barbs are disunited and placed at a 



distance, and they are often not at 



all unlike hairs in appeaiunce. The 



feathers of Birds not only greatly 



conduce to the high temperature of 



the body, but also serve to keep out 



moisture, to which end thtre is a, 



peculiar oil-gland at the base of the 



tail, with the secretion of which the 



bird anoints its plumage. 



The skeleton of Birds exhibits many 

 points of peculiar interest, mostly in 

 adaptation to an ^grialjnode of life ; 

 but only some of the more important 

 of these can be noticed here. The 

 entire skeleton is at the same time 

 peculiarly compact and singularly 

 light, the compactness being due to 

 the presence of an unusual quantity 

 of phosphate of lime, and the light- 

 ness to the fact that many of the 

 bones are filled with air in place of 

 marrow. The cervical region (neck) 

 of the vertebral column is unusually 

 long and flexible, since the fore-limbs 

 are useless as organs of prehension, 

 and all these functions have to be 

 performed by the beak. In all Birds 

 the neck is, at any rate, sufficiently 

 long to allow of the application of 

 the beak to the tail, so as to permit 

 of the cleaning and oiling of the 

 whole plumage. The vertebrES which 

 form the back or dorsal region of the 

 spine are generally more or less im- 

 movably connected together, so as to 

 give a base of resistance to the wings. 

 In the Ostrich, however, and in other 

 birds, in which the power of flight is 

 either very limited or is absent, the 

 dorsal vertebrae are more or less movable one upon the other. The 

 vertebrae which follow the dorsal region of the spine are all amalga- 

 mated together to form a single bony mass, which is termed the 



Fig. 201.— Quill-feather (Stenopsls) : a 

 Quill or barrel ; S Shaft ; c o Webs, 

 comijosed of the barb-s, and to- 

 gether forming the '* vane." 



