THE FEATHERS OF BIRDS. 235 



The exoskeleton of Birds consists almost entirely of epi- 

 dermic structures in the form of horny sheaths, scales, plates, 

 or feathers. No bird possesses dermal ossifications, unless the 

 spurs which are developed upon the legs and wings of some 

 species may be regarded as such. 



The feathers are of various kinds. Those which exhibit 

 the most complicated structure are called pennce, or contour 

 feathers, because they lie on the surface and determine the 

 contour of the body. In every penna the following parts are 

 to be distinguished : A main stem (soapus) forming the axis 

 of the feather, and divided into a proximal hollow cylinder, 

 partly imbedded in a sac of the derm, called the calamus, or 

 quill ; and a distal vexillum, or vane, consisting of a four-sided 

 solid shaft, the rachis, which extends to the extremity of the 

 feather, and bears a number of lateral processes, the barbs. 

 The calamus has an inferior aperture {umbilicus inferior), into 

 which the vascular pulp penetrates ; and a superior aperture 

 {umbilicus superior) situated on the under-surface of the 

 feather at the junction of the calamus with the scapus. The 

 barbs are narrow plates, tapering to points at their free ends, 

 and attached by their bases on each side of the rachis. The 

 edges of these barbs are directed upward and downward, 

 when the vexillum of the feather is horizontal. The inter- 

 stices between the barbs are filled up by the barbules,' pointed 

 processes, which stand in the same relation to the barbs, as 

 the barbs do to the rachis. The barbules themselves may be 

 laterally serrated and terminated by little hooks, which inter- 

 lock with the hooks of the opposed barbules. In very many 

 birds each quill bears two vexilla; the second, called the 

 aftershaft {hyporachis) being attached on the under side of 

 the first close to the superior umbilicus. The aftershaft is 

 generally much smaller than the chief vexillum ; but in some 

 birds, as the GasuaridoB, 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 pennae, in having the barbs 

 soft and free from one another, when they constitute penno- 

 plumoB, or plumuloB (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 Ratitoe, the Penguins, and some 

 others. Generally, the pennse are arranged in definitely cir- 



