420 



AVES. 



The most important feature in the external appearance of birds 

 is their covering of feathers. The skin is naked in a few places 

 only — as on the beak, the cere, the toes, (with a few exceptions, 

 Lagojrus, etc.), usually on the tarsometatarsus, sometimes on 

 the neck (vulture), or even on the abdomen (ostrich), and on the 

 cutaneous outgrowths of the head and neck (gallinaceous birds, 

 vulture). Wliile the cere is soft, the edges of the rhamphotheca 

 are usually cornified, and are only exceptionally soft (ducks, 

 snipe), and are then richly innervated, serving as a fine tactile 

 organ. The skin on the toes and metatarsus is cornified so as 

 to form a firm horny covering, which is sometimes granular, 

 more often divided into scales, and which may afford important 

 systematic characters. When this integument is scaly in front 

 and smooth behind, the metatarsus is said to be laminiplaritar 

 (thrushes and other Oscines). The following special horny 

 structures may be mentioned : the claws on the toes (and 

 sometimes on the first and second digit of the manus), the spurs 

 on the posterior and internal edge of the metatarsus in the male 

 Gallinaceae, and on the carpus (some Charadriidae, etc.). 



Feathers are closely allied to scales. On the wings of pen- 

 guins the small feathers present are hardly distinguishable from 

 scales. They arise by the cornification of the epidermis of 

 papillae containing a vascular core. These papillae at first 

 joroject freely on the surface, therein differing from hairs, but 

 they very soon become secondarily enveloped in a pit (follicle) 

 which gradually deepens as the development continues. 



A typical feather consists of the following parts : There is a 

 stiff axial rod, the scapus or stem, running the whole length of 

 the feather. This consists of two parts ; the proximal, hollow, 

 semitransparent calamus or quill, and a distal part, the shaft or 

 rhachis. The calamus is cylindrical, is partly embedded in the 

 skin, and encloses the dried up vascular papilla of the growing 

 feather ; at its proximal end is a small opening, the inferior 

 umbilicus, and at its distal end where it passes into the rhachis 

 there is on the ventral side, i.e. on the side adjacent to the body, 

 a second opening, the superior umbilicus. The rhachis is solid, 

 somewhat quadrangular, and grooved on its ventral surface ; it 

 carries a nuniber of lateral processes, the barbs {rami), which 

 again carry still smaller processes the barhules {radii). The 

 barbs and barbules constitute the vane {vexillum) or web of the 



