THE EXTBENAIi STEITCTUBB OE BIEDS. 1 35 



nary," and "generative" systems, as well as organs "of investment 

 and support," which consist of the external skeleton — ^the skin 

 with its appendages — and the internal skeleton, or the skeleton 

 commonly so called. 



We will first notice the organs of investment and support, 

 beginning with the external skeleton — the sMn and its appen- 

 dages. 



The skin of a Bird, like our own, consists of two layers :— an 

 external layer called the epidermis or ecteron, and a .deep fibrous 

 layer, supplied with nerves and blood-vessels, called the dermis 

 or enderon. The feathers, the horny scaly outer skin of the feet 

 and legs, and the outer covering of the beak are all epidermal 

 structures — that is appendages or modifications of the epi- 

 dermis. 



Before describing any of these, we will first note some of 

 the general conditions which modify and determine the leading 

 characteristics of bird-structure. 



The whole organization of Birds is specially modified to. sub- 

 serve flight. It is this which mainly governs the general shape 

 of the body, the arrangement of the organs of movement — the 

 muscles — and the position and packing of the internal organs or 

 viscera, which are so placed as to be most conveniently carried 

 near the centre of the body's gravity, so helping to maintain 

 that in a suitable position. FUght determines the structure 

 and form of the most internal organs, the bones, as well as of 

 those wonderfully delicate and beautiful structures which clothe 

 the body externally. At the same time all Birds have more or 

 less to walk, and very many have to swim ; and hence arise various 

 structural conditions which, however, with rare exceptions, are 

 never so modified as to impede flight. 



The fore limbs are absolutely and entirely given up to flight, 

 not even taking on any other function in birds which cannot fly. 

 Those of the Ostrich only assist its running by their flapping, 

 while those of the Penguin act as organs of aquatic flight 

 beneath the surface of the water. 



Since, then, some members must be applied to ordinary 

 locomotion, all birds require to have a pair of legs exclusively 

 or mainly devoted to that function. But as much delicate 

 work (as e. g. in nest-building) has often to be done, which 

 a foot even when used like a hand, as by a Parrot, could 

 never do, there is nothing but the head to do it. Hence 

 arises the need of a more or less elongated and very moveable 



