262 VERTEBRATE ZOOLOGY 



a larger than usual head for muscle attachment. The wrist, hand, and 

 finger bones are highly modified both through loss of whole bony 

 units and by the fusion of the remaining bones into strong complexes. 

 The thumb or poUex is reduced to a small rudiment, the index finger 

 is the largest, the second finger fairly well developed, but there is no 

 trace of the third and fourth fingers. The phalangeal part of the fore 

 limb is reduced essentially to a one-fingered condition. 



Of the wing muscles those of the upper arm are very large and 

 powerful, those of the lower arm much reduced, and those of the hand 

 atrophied. The only movements of the wings are those of elevating, 

 depressing, extending and flexing. The real flight muscles are the 

 chest muscles or pectorals, massive groups of fine-grained striated 

 fibers, which are inserted upon the keel of the sternum. These muscle 

 masses, which are capable of prolonged exertion without fatigue, 

 correspond to the cylinders of the aeroplane motor. 



The wing feathers are the main factors in giving large planing surface 

 to the wing. A feather (Fig. 143) from the morphological standpoint, 

 is no more nor less then an elaborately subdivided scale, rolled up 

 into a cylinder proximally and expanded into a flat vane at the distal 

 end. The quill is residue of the embryonic rolled-up stage. The vane 

 is composed of a number of subdivisions called barbs, each of which is 

 redivided into minute barbules which are hooked to the barbules of 

 adjacent barbs so as to give stability to the whole vane and to make 

 the feather as a whole a coherent, springy plane. A single row of large 

 flight feathers grows out from the back of the arm and hand bones, 

 each partly overlapping its neighbor. Several rows of so-called coverts 

 overlie these like shingle rows. The overlapping arrangement of all 

 the feathers contributes greatly to make the wing a fairly rigid, but 

 sufficiently flexible plane, which is better adapted for the purpose 

 than the perfectly rigid planes of man-made machines. The wing 

 differs also from the plane in that it is jointed and capable of 

 being folded away when not in use, or of regulating its exposed sur- 

 face by flexures. 



2. Power. — The secret of great and sustained power lies in the 

 capacity to convert chemical energy into mechanical motion through 

 rapid and complete combustion of fuel. In the aeroplane, gasolene is 

 the fuel, the electric spark is the combustion agent and oxygen the 

 combustor; in birds carbohydrates, etc., constitute the fuel, the nerve 

 impulse is the combustion agent and oxygen the combustor; the 



