CHAPTER XIX. 



GENERAL ACCOUNT OF THE SKELETON IN 

 BIRDS. 



EXOSKELETON. 



The epidermal exoskeleton of birds is very greatly de- 

 veloped, feathers constituting its most important part. 



Three kinds of feathers are found, viz. (a) pennae including 

 quills and coverts, (6) down feathers or plumulae, and (c) filo- 

 plumes which are rudimentary feathers. The structure of 

 the different kinds of feathers is described on pp. 303 306. 



Sometimes a fourth class of feathers, the semiplumae, is 

 recognised. They have the stems of pennae, and the downy 

 barbs and barbules of plumulae. 



In most birds the pennae are riot uniformly distributed 

 over the whole surface of the body, but are confined to certain 

 tracts, the pterylae ; while the intervening spaces or apteria 

 are either bare or covered only with down feathers. In some 

 birds, however, such as the Ratitae a*nd the Penguins, pennae 

 are evenly distributed over the whole body. 



In many birds the calamus or quill bears two vexilla or 

 vanes, the second of which, called the attershaft or hypo- 

 rachis, is generally much the smaller, and is attached to the 

 under surface of the main vexillum. In the Moas, Emeu and 

 Cassowary the two vexilla in the adult bird are nearly equal 

 in size ; though in the nestling Emeu one is much longer than 

 the other. The aftershaft is very small in most Passeres and 

 gallinaceous birds, but is comparatively large in Parrots, Gulls, 



