THE BIRDS 263 



of the aerial life they normally lead. The fore-limbs are always 

 modified into organs of flight, even when, as in some cases, they are 

 merely rudimentary ; and while the bones of a bird's wing correspond 

 in a general way to the bones of the fore-legs of a mammal, as 

 might be expected, the bones, the joints and muscles show consider- 

 able modification as compared with the fore-limbs of other animals. 

 The problem of flight, that has only of recent years engaged our 

 attention, was solved by Nature in various ways before man ever 

 came into existence. The bird, the bat, the insect, are all so many 

 examples of efficient flying machines, each perfect in its way, 

 though differently constructed, and man has yet a long way to 

 go before his mechanical constructions approach in proficiency 

 any one of these natural aeroplanes. 



A bird when it flies has to work against its weight, hence the 

 muscles employed to pull down the wing against the air, and so 

 raise the body, are remarkably large and strong. The principal 

 muscle used in this manner is the great pectoral muscle attached 

 to the keel or ridge (which is a distinguishing feature of the breast- 

 bone of a flying bird), and inserted into the humerus or arm- 

 bone ; and the bird opens out its wing by straightening the elbow 

 and wrist-joints, and folds it by bending the elbow and bringing 

 forward the wrist- joints. 



The feathers which clothe the bird correspond in their nature 

 to the scales of reptiles and the hairs of mammals, and are modifica- 

 tions of the cells of the epidermis, or upper skin, and the cutis, or 

 under skin. The feathers are not all of one kind ; the two most 

 familiar types are the contour feathers, which form the outer 

 covering of the bird, and the down feathers, which form a soft, 

 dense underclothing corresponding to the under-fur of the mammals. 



A contour feather consists of a quill, or calamus; a shaft, or 

 rhachis ; barbs, or rami ; and barbules, or radii. The shaft is a 

 continuation of the quill, the whole forming the axis of the feather. 

 The quill is a hollow cylinder within which is a series of oblong, 

 hollow cells ; containing the nutrient material, or pulp, from which 

 the feather is derived ; it forms the lower part of the axis, the base 

 being inserted in the skin. The shaft of the axis is solid, and 

 bears on each side a broad, elastic web, made up of a series of 

 narrow, tapering plates, or lamina the barbs set obliquely on 

 the shaft. The barbs, in turn, bear a double row of smaller pointed 



