114 INTRODUCTION TO ZOOLOGY. 



to scales. That is, the same thing Avhich is devel- 

 oped on the fish as scales is developed on tlie l)ird as 

 feathers. 



Among the feathers will be found hairs. The 

 hairs are scattered among the feathers, being clustered 

 together in some instances around the mouth and 

 ears. Since they have the same position as feathers, 

 we are compelled to believe that feathers and hairs 

 are homologous structures. 



This covering of the body which may be devel- 

 oped as scales, feathers, or hairs, constitutes what 

 may be called the exoskeleton. It is composed of a 

 substance called keratin, one ingredient of which is 

 sulfur. A pigeon, then, has really two skeletons, an 

 exo- and an endo- skeleton, although when we speak of 

 the skeleton of a bird we always mean the endoskel- 

 eton. 



The wings are higlily specialized organs, adapted 

 for flight. The large feathers, the primaries, are ar- 

 ranged in such a way that the short barbs of one 

 primary overlap the longer and less stiff barbs of tlie 

 other side of the next feather. This arrangement 

 makes a firm surface for the downward stroke, but 

 allows the passage of air between the feathers much 

 more readily when the wing is rising. 



The muscles that move the wing are very curious. 

 Two muscles, pectoralis major and pectoralis minor, 

 lying side by side, pi'oduce directly opposite motions 

 of the wing when tliey contract altornately. The ten- 

 don of the pectoralis minor passes directly over the 

 end of the humerus, and is attaclied to the upper side 

 of the same bone. This constitutes a pulley, and 

 changes the direction of the motion. It pulls the 

 wing uj^ward. Tlie pectoralis major passes over no 

 pulley, so its contraction pulls the wing downward. 



The pectoralis major is much larger than the pec- 

 toralis minor. This is readily associated with the fact 



