Glass 5. Aves. 



431 



limbs are never adapted for terrestrial locomotion, but, with few 



exceptions, for flight. The body is usually borne upon the hind 



limb's in a semi-erect position ; it rests only upon the toes, not on 



the very slender metatarsus. The 



neck is of considerable length and 



very movable, the trunk is short; 



the tail in all existing Birds is 



short, but the large steering 



feathers* give it an appearance 



of greater length. The face portion, 



the beak, is peculiarly modified; 



it is usually elongate, and covered 



with a horny sheath. 



The skin is almost entirely 

 covered with feathers, com- 

 plicated appendages consisting of 

 cornified epidermis cells. They 

 arise as dermal papillae but soon 

 sink into sac-like pits in the skin, 

 the feather follicles; the feather 

 develops from the epidermal layer 

 covering the papilla.t Feathers 

 exhibit a great diversity of form. 

 The contour feathers {pennce) 

 are firm, the distal parts at least 

 reaching the surface and forming 

 the outer contour of the bird (in 

 distinction to the down which 

 lies below) ; they consist of the 

 following parts : the proximal 



portion is a short, cylindrical, hollow quill {calamus) which is 

 imbedded in the feather-follicle, a more or less deep 

 dermal pit. The quill is continued into the shaft {racMs) which 

 consists externally of a hard cuticle, within of a loose horny 

 mass; and is thinner at the tip. (Quill and shaft together form 

 the stem \_sca'pus'\ of the feather.) From the shaft there arises on 

 ■each side a series of barbs which are again furnished with 

 barbulesj the barbs and shaft together, are termed the vane 

 [vea-illum) . At the distal portion of the shaft, and this may form 

 a larger or smaller portion of the feather, the barbs are stiff and 

 compressed (their flat surfaces turned towards those of neighbouring 



lig. 357. Portion of a feather: 

 diagrammatic. s shaft, a barb, st bar- 

 bules of an anterior row with hooks, st' 

 barbixLes of posterior rows ; the former 

 cover the latter and seize them by means 

 of the hooks on the edges. — Orig. 



* When, in the description of Birds, a long or short tail is mentioned, this 

 always refers to the length of the steering feathers. 



t It is, however, only the first phunage of young Birds, which develops as papillae 

 on the surface ; the later feathers arise as similar papiUse, which lie, however, at the 

 base of the follicle of the preceding feathers. 



