BIRDS 141 



an additional support from the collar-bone (see Part L, p. 11). In birds, 

 on the other hand, the joint is placed at a point where three bones meet. 

 The first of these bones is the shoulder-blade, or scapula, which lies close 

 against the thorax; the second is the coracoid, which connects the 

 scapula with the sternum ; and the third is the collar-bone, or clavicle. 

 The two clavicles are anchylosed with one another at their anterior 

 extremities, forming a V-shaped bone, the " furcula," popularly known as 

 the "merry-thought." In this manner the clavicles assume the form of 

 a bent rod of great elasticity, an arrangement of the highest importance 

 to the avian body. The function of this furcular arch is to hold off from 

 the thorax, and thence from the internal organs (lungs, heart, etc.), and 

 to transfer to the scapular joint, the extra pressure which would other- 

 wise fall on these parts in the simultaneous downward stroke of the 

 wings. This action may be exemplified by a bridge, in which the pressure 

 is transferred to the buttresses of the supporting pillars. (How do mam- 

 mals as a rule move their fore-limbs ?) 



2. Tail. A long tail, such as is found in most mammals, would to a 

 bird entail an unnecessary load, and, moreover, would be much too difficult 

 to manage as an aerial rudder. Hence the tail is always short and 

 enclosed within the integument. Its terminal portion, which is formed 

 by the anchylosis of several vertebrae, is almost always of considerable 

 size, and supports the rectrices, or steering feathers (see Section 10, a). As 

 it is necessary that the rudder should be movable, the vertebrae which 

 immediately precede this terminal portion are freely movable on one 

 another, and do not anchylose like those of the trunk. 



3. Fore-limbs. The forward movement of a bird is effected by the 

 stroke of the wings, as that of a boat by the oars. The wings, in fact, 

 form the oars of an air-boat or balloon (see Section 11). Of all modes of 

 motion, however, flight is the most difficult, for not only has the flying 

 animal to perform appropriate movements in order to progress forwards, 

 but has also, while "swimming" through the air, to carry the whole 

 weight of its body. (Why ? Compare with terrestrial and aquatic 

 animals ; and see the whale.) Accordingly, the bird must be endowed 

 with great poiver in its organs of flight ; this necessitates powerful muscles 

 and large bony surfaces for their attachment. These are provided by the 

 bones of the shoulder-girdle, and especially by the long, broad, and shield- 

 like sternum, the surface of which is further increased by a remarkably 

 large bony keel, or crest. (Compare, however, the ostrich. Why may 

 this crest be comparatively thin ? Consider that the wings are depressed 

 simultaneously.) These large muscles of the thorax, however, do not 

 merely act as the motor engine of the living air-ship, but, in combination 

 with the heavy viscera (see stomach), also supply its ballast. Con- 



10 2 



