THE EXTERNAL STRUCTURE OF BIRDS. 135 
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 skin 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. Flght 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 g given up to flight, 
not even taking on any other function in birds which cannot “fly. 
oe 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 ¢.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 

