BIRD. 



455 



pieces of mechanism, as compared with this portion of 

 the wing. They are strong in one direction only; 

 that is, when the strain or pressure is upon the out- 

 side of the bend, and this is in them the position of 

 least stability. If its position is reversed, the bow 

 bridge is weaker than a straight plank by the whole 

 extent to which it is bent, as the string offers no 

 resistance to further bending. But it will be seen, 

 on looking back at the figure, that the radius, which 

 answers to the bow, is one of very curious curvature. 

 Though less in diameter at the middle than toward 

 the ends, it has its stiffness there greatly increased by 

 being not only straight, but a very little bent the other 

 way. This against a single strain would give it the 

 same strength as a bow has by being thicker, and con- 

 sequently bending less toward the middle than toward 

 the ends. But the bone has to bear strains and twists 

 in all directions ; and therefore, though the double 

 curvature cannot be shown in a figure, it will be 

 found, upon examination of the bone itself, that some 

 part of it is a bow in what direction soever it is turned. 

 The ulna is also both a tie and a strut, and resists 

 equally the drawing asunder and the pressing together 

 of the ends of the larger bone. It is nearly straight, 

 which is the form of greatest strength against both of 

 these ; and it is spindle-shaped, or thickened toward 

 the middle of its length, which is the form of greatest 

 stiffness. But we must not proceed much farther in 

 the consideration of this exquisite piece of mechanism : 

 it would not only afford study for years, but 

 remain in great part an unexplained wonder after we 

 had exercised the longest life upon it. 



But, admirable as is the structure of this moving 

 part of the wing, it would fail in producing its office 

 if the point to which it is attached, and the organs by 

 which it is moved, did not partake of the same 

 character -. for without this it would be like a well- 

 made tool or a powerful weapon placed in an unskilful 

 hand. 



The sternum is the grand bone upon which the 

 articulation of the wing at the shoulder-joint, and the 

 muscles which move that joint, are founded; and there- 

 fore, the general power of every wing may be judged 

 of more from that bone than from any other single part. 



But, in order to have a clear understanding ot the 

 relative power of wings, it is necessary to examine 

 carefully all the bones which support the point to 

 which the wing is articulated, or which afford insertion 

 to the muscles by which it is moved. It has been 

 already mentioned, in the general notice of the struc- 

 ture of birds, that the three bones which support that 

 point are the coracoid, the scapula, and the branch of 

 the furcal. The articulation is more immediately 

 upon the coracoid, though the shallow socket in 

 which the head of the humerus plays, is often in 

 part formed by the scapula, with the arm of the fur- 

 cal bones against the other two on the inside, beyond 

 the socket, and as far as their extremities ; and imme- 

 diately behind the head of the coracoid, to the outside 

 of which the scapula is applied, there is a foramen 

 or hole, which serves as a pulley, through which the 

 tendons of the muscles that raise the wing pass. The 

 firmness with which these three bones are united to 

 each other is always in proportion to the power of 

 flight in the bird. In no bird of powerful wing can 

 any of them be dislocated from the other two without 

 fracture ; and in old eagles and hawks, the portion of 

 cartilaginous matter by which they arc united passes 

 uinplctely into bone, though bone which remains 



a little flexible, that they cannot be detached from 

 each other by maceration in water. 



It is worthy of remark, as affording a beautiful 

 instance of that provision of nature by means of which 

 the vital parts of animals are always protected from 

 injury even in their most violent actions, if those 

 actions are natural, or essential to the accomplish- 

 ment of those purposes which their Creator has 

 ordained them to accomplish, that the anterior extre- 

 mities of no vertebrated animal are articulated upon 

 the spinal column. This is very remarkably the case 

 in birds of very powerful wing. 



In mammalia, the articulation of the humerus is 

 always upon the scapula only; they want the coracoid 

 bones entirely, and in bears, and other species which 

 grasp or hug between the fore legs, the clavicles are 

 wanting. Even in those species, as in the quadru- 

 mana, which have the clavicles most perfectly deve- 

 loped, the office of these bones is secondary, chiefly 

 that of keeping the shoulder-joints apart from each 

 other, so that the animals may stretch their fore-legs, 

 or arms, at right angles to the mesial plane of the body, 

 and not use them parallel to that plane only, as is the 

 case with those mammalia which employ the forelegs 

 only in walking. 



In the mammalia, the scapular bone is flattened 

 and extended to the shape of a triangular plate, and 

 it is furnished with a septum, or elevated keel of bone, 

 to each side of which there are muscles attached, 

 which move the bone backwards or forwards, accord- 

 ing as is necessary to the action of the other parts of 

 the limb. But in birds the scapula is never furnished 

 with a ridge or keel of this description ; and it is most 

 developed and enlarged at the end farthest from the 

 joint in those birds which make the nearest approach 

 to the quadruped action of the mammalia, as for 

 instance, in the grebes, which use the wings and the 

 feet, with nearly equal energy, as they make their 

 way under water. In those land-birds, such as the 

 ostrich, which cannot raise the wings, but use them 

 only for balancing the body as they run, the scapulae 

 are less developed ; but even in them, they are more 

 so than in the birds of the most powerful flight. These 

 birds have not the clavicles united in front into a 

 regular arch, as in powerfully-winged birds, or even 

 into a fork with the branches nearly straight, as in the 

 gallinidae and other bad fliers ; and in some of the 

 species they are little else than mere processes on the 

 anterior edges of the coracoid bones. 



In no case, however, of these short-winged birds 

 are the coracoid bones, which connect the scapulars 

 with the anterior part of the sternum, wanting : and 

 in no case, even of those of most vigorous flight, in 

 which the furcal bone is the most developed, are the 

 humeral bones of the wings articulated directly upon 

 that bone. 



It is here, that we find one of those strikingly dis- 

 tinctive characters from which we can at once pro- 

 nounce a bird to be a bird, whether it has or has not 

 the power of flight ; and here, that we are enabled 

 to see which is the characteristic bone in this part of 

 the skeleton which is possessed by birds and never by 

 mammalia. On this point it would be unjust not to 

 acknowledge the merit due to the science and the 

 sagacity of Cuvier. Before his time, those who 

 treated of the structures of animals, and of the struc- 

 'tural differences of the several classes of vertebrated 

 ones, were somewhat at a loss which to consider as 

 the additional bone in birds, and which as the true 



