254 VERTEBRATE ANIMALS. 



one another by means of the barbules, the ends of which are 

 hooked. Toward the base of the shaft, however, the barbs 

 are usually more or less separate and placed at a distance from 

 one another, constituting 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 appearance. 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 there 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 aerial mode 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 pres- 

 ence of an unusual quantity of phosphate of lime, and the 

 lightness to the fact that many of the bones are filled with air 

 in place of marrow. The cervical region (neck) of the verte- 

 bral 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 vertebrae which form the back or 

 dorsal region of the spine are generally more or less immov- 

 ably 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 amalgamated together to form a single bony 

 mass, which is termed the " sacrum," and this, in turn, is 

 united on both sides with the bones which form the pelvic 

 arch, which carries the hind-limbs. The vertebrae of the tail 

 are more or less movable upon one another ; and in almost all 

 living birds, when fully grown, the last joint of the tail (Fig. 

 129, B, s) is a long, slender, ploughshare-shaped bone, which 

 is really composed of several vertebrae united together. It is 

 usually set on at an angle nearly perpendicular to the axis of 

 the body, and it serves to support the great tail-feathers, 

 which act as a rudder during flight. It also serves to support 

 the oil-gland, which supplies the secretion with which the 

 feathers are lubricated. The skull in birds has its several 



