BASE OF THE WING. 57 



that it keeps the heads of the coracoids and scapu- 

 lars in their proper places, meet each other. The 

 shoulder-joint, or articulation of the humerus, which 

 is the centre of the grand action of the wing, is 

 thus placed on the firmest of all supports a tripod, 

 which is the only number of supports that will form 

 a sure base upon all kinds of surfaces. In birds of 

 the most powerful wing, these three supports divide 

 the space round the articulation into nearly three 

 equal parts, and they do not differ much in strength ; 

 they all "give" a little, so that the joint is not 

 nearly so liable to dislocation as if the point to 

 which it is articulated were fixed ; neither is there so 

 much danger of fracture or of concussion to the vital 

 parts by any sudden jerk given violently to the wing. 



It is one of the most beautiful parts of the struc- 

 ture of animals, that those organs which have to per- 

 form the most violent motions are never directly arti- 

 culated on an immoveable base, or a base imme- 

 diately in contact with the spinal column, far less 

 with the bones of the head. There is always the 

 play of a slow-moving joint, or union of some 

 description or other, between the articulation of 

 the moveable bone and that bone which encloses 

 the nervous mass. By this means that mass is made 

 to ride smoothly while the animal leaps, or bounds, 

 or flies, or otherwise acts powerfully and irregularly; 

 just as the springs of a carriage enable those within 

 it to ride smoothly, notwithstanding the jolting of 

 the wheels upon an uneven road. 



The scapula, or blade-bone, has less motion in 

 birds than in mammalia, because, with the exception 

 of those mammalia which have flying membranes, 

 the motion of fore-legs is crosswise to that of wings. 

 In the bat tribe there is a slight approximation to 



