THE FEATHERS OF BIRDS. . 275 



close to the superior umbilicus. The af tershaf t is generally 

 much smaller than the chief vexillum ; but in some birds, 

 as the Casuaridce, the two are of equal size, or nearly so. 

 Muscles pass from the adjacent integument to the feather 

 sac, and by their contraction erect the feather. The other 

 kinds of feathers differ from the pennse, in having the barbs 

 soft and free from one another, when they constitiite pemio- 

 plumce, or plumulce (down), according as the scapus is much 

 or little developed. When the scapus is very long, and the 

 vexillum very small or rudimentary, the feather is termed 

 a jilopluma. 



The contour feathers are distributed evenly over the body 

 only in a few birds, as the RatitcB, the Penguins, and some 

 others. Generally, the pennae are arranged in definitely 

 circumscribed patches or bands, between which the integu- 

 ment is either bare, or covered only with down. These 

 series of contour feathers are termed pterylai, and their 

 interspaces, apteria. 



In some birds, such as the Herons, plumule of a peculiar 

 kind, the summits of which break off into a fine dust, or 

 powder, as fast as they are formed, are developed upon, 

 certain portions of the integument, which are termed powder 

 down patches. 



The integument of birds is, for the most part, devoid of 

 glands ; but many bii-ds have a peculiar sebaceous gland 

 developed in the integument which covers the coccyx. This 

 uropygial gland secretes an oily fluid, which the bird 

 spreads over its feathers by the operation of " preening." 

 The excretion passes out by one or two apertures, commonly 

 situated upon an elevation, which may or may not be 

 provided with a special circlet of feathers. 



In various birds (e.g., the Turkey) the integument about 

 the head and neck develops highly vascular and sometimes 

 erectile processes (combs, wattles). 



The spinal column of birds contains numerous and well 

 ossified vertebras, a considerable number of which (more 

 than six) are ankjiosed together to form a sacrum. Of 



