No one can have examined attentively the bony framework whereby the body of a bird is sustained, without being forcibly impressed with the lightness as well as the compactness of its construction. The most wonderful economy is exhibited in the arrangement of the weighty material of which it consists. The bones present in their interior extensive cavities, whereby they are considerably lightened, and their walls, although exceedingly dense and strong, are much thinner than in any other animals. The extremities of the cylindrical bones are occupied by a light open network of slender filaments shooting across in every direction from wall to wall, and as these attenuated buttresses are likewise hollow, it is easy to perceive how incomparably lightness and strength are here conjoined. The extent to which the skeleton is thus filled with air varies in different birds in relation with their powers of flight. In the Swifts and the Humming-birds every bone of the skeleton, even to the toes and the claws, is permeated by the atmospheric fluid. In the opposite extreme, the terrestrial Apteryx and the aquatic Penguin have not a single bone thus excavated. [Illustration: Fig. 12.--SKELETON OF A BIRD. _a_, the Skull; _b_, Vertebræ of the Neck; _c_, Dorsal Region of the Spine; _d_, Vertebræ that support the Tail; _e_, _e_, _e_, the Sternum or Breast-bone; _f_, _f_, _f_, _f_, the Ribs; _h_, the Scapula, or Shoulder-blade; _i_, _i_, the conjoined Clavicles or Collar-bones, forming the Furculum, or "Merry-thought;" _k_, the Coracoid Bones; _l_, the Humerus, or Arm-bone; _m_, _n_, Bones of the Fore-arm; _o_, _b'_, Bones of the Wrist or Carpus; _p_, _p_, the Metacarpal Bones and Rudimentary Thumb; _q_, _q_, Pieces representing the Middle Finger; _r_, _s_, _s_, the Pelvis, formed by the consolidation of the hinder Vertebræ of the Back into one piece; _t_, the Thigh-bone; _u_, the Leg; _x_, _x_, the Tarso-Metatarsal Bones, usually called the Tarsus; _y_, the Hallux, or Hinder Toe; _z_, _z_, the Front Toes. ] The skeleton of an animal formed for flight must be constructed upon mechanical principles of a very refined character. The utmost lightness is indispensable; nevertheless, in a framework which has to sustain the powerful action of muscles so vigorous, strength and firmness are equally essential. It is in combining these two opposite qualities that the human mechanician exhibits the extent of his resources and the accuracy of his knowledge; but let the best and most ingenious mechanic carefully examine the skeleton of a bird, and we doubt not that in its construction he will find all his ingenuity surpassed, and perhaps derive not a little instruction from the survey.