AYES. 



The muscles of the thorax are powerfully developed in connection 

 with the power of flight, and a peculiar muscular arrangement, in 

 consequence of which the toes are mechanically bent when the animal 

 sits, deserves mention. 



Exoskeleton. The most important character in the external ap- 

 pearance of Birds is their covering of feathers. The skin is only 

 naked in a few places as on the beak, the toes, usually on the tarso_ 

 metatarsus, and sometimes on the neck (Vulture), or even on the 

 abdomen (Ostrich); and also on the cutaneous outgrowths of the 

 head and neck (Gallinaceous birds and Vulture). While the naked 

 skin at the base of the beak is soft (so-called cere), the edges of the 

 beak are usually cornified, and are only exceptionally soft (Ducks, 

 Snipe), and then are richly innervated and serve as a fine tactile 

 organ. The skin on the toes and tarso-metatarsus is also 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 forms a long con- 

 tinuous horny sheath on the anterior and lateral surfaces of the 

 metatarsus, the latter is termed laminiplantar (Thrushes and singing- 

 birds). The following special horny structures may be mentioned 

 the claws on the toes, the so-called spurs on the posterior and internal 

 edge of the metatarsus in the male Gallinacece, and sometimes on the 

 thumb-joint of the wing (Parra). 



The feathers of Birds correspond to the hairs of Mammalia, and 

 like them arise in pits of the derinis lined by the epidermis. At the 

 bottom of the pit there is a vascular papilla, the epidermal invest- 

 ment of which gives rise by its rapid growth to the first rudiment of 

 the hair or feather, around which the epidermal lining of the pit 

 lies like a sheath. In the feather the axial part or stem with the 

 calamus and shaft (rhachis) is to be distinguished from the vane 

 (vexillum). The cylindrical hollow calamus is partly embedded in 

 the skin, and encloses the dried-up papilla (pith) ; the rhachis is the 

 projecting part of the stem, and bears a number of lateral processes 

 the barbs which with their attached parts (barbules) constitute the 

 vane (vexillum). The lower slightly concave side of the rhachis 

 presents along its whole length, from the end of the calamus to 

 the point of the feathers, a deep longitudinal groove, at the base 

 of which a second feather the after shaft (hyporhachis) arises, 

 which, as well as the main-shaft, gives off two rows of barbs, but 

 only in rare cases (Cassowary) reaches the length of the main shaft, 

 and is sometimes (wing and tail quills) completely absent. The barbs 



