AYE'S. 



285 



main support to the wing, and the great point 

 of resistance to the humeri during the down- 

 ward stroke of this aerial oar. The superior or 

 humeral end of this bone is commonly bifur- 

 cate ; the outer process is the strongest, and 

 completes the glenoid cavity anteriorly, (I, 

 Jig. 1 30,) above which it rises, to a greater or less 

 extent, and affords, on its inner side, an arti- 

 cular surface for part of the acromial end of the 

 clavicle : the inner process is short and com- 

 pressed, and is also joined by ligament to the 

 acromial end of the clavicle. Just below the 

 origins of these processes an articular surface 

 extends transversely across the posterior part of 

 the coracoid bone by which it is firmly united 

 by fibro-cartilaginous substance to the scapula. 

 The glenoid cavity resulting from the union of 

 these two bones is not, however, always equal 

 to the reception of the entire head of the hu- 

 merus. In the birds, which Mr. Vigors re- 

 gards as composing the typical orders of the 

 class, viz. the Rap tores and Insessores, (the 

 aves aerete of Nitzsch,) a small but distinct bone 

 extends between the scapula and coracoideum 

 along the superior part of the articular cavity for 

 the humerus, which it thus completes. Nitzsch, 

 the discoverer of this element of the scapular 

 apparatus, denominates it the capsular bone, 

 (Schulterkapselbeine) ; by Meckel it is called 

 the Os humcro-scapulare, and is regarded as 

 the analogue of the scapula inferior of reptiles. 

 In the Aberrant orders of birds, as the Rasoi^es, 

 Grallatores, and Natatores, there is, in place 

 of this bone, a strong elastic ligament or fibro- 

 cartilage extended between the scapula and 

 coracoideum, against which that part of the 

 head of the humerus rests, which is not in con- 

 tact with the glenoid cavity. 



The clavicles (v, fg. 125, b, Jig. 130) in 

 birfls, as in the mammalia, are the most variable 

 elements of the scapular apparatus. In the 

 Ground Parrots of Australia ( Pezophorus, II- 

 liger) they are rudimentary or wholly deficient ;* 

 they are represented by short processes in the 

 Emeu, Rhea,and Cassowary; they do not come 

 in contact inferiorly in the Ostrich, although 

 they reach the sternum. In the Toucans they are 

 separate, and do not reach the sternum. In the 

 Hornbills and Screech Owl (Strix ulula) they 

 are united at their inferior extremities by carti- 

 lage. In the rest of the class they are anchylosed 

 together inferiorly, and so constitute one bone, 

 thefurculum, or merrythought. From the point 

 of union a compressed process extends down- 

 wards in the Diurnal Raptores, the Coniros- 

 tral Insessores, the Rasores, most of the Gral- 

 latores, and Natatores, in which a ligament 

 extends from its extremity to the ento-sternum. 

 The process itself reaches the sternum, and is an- 

 chylosed therewith in the Pelicans, Cormorants, 

 Grebes, Petrels, and Tropic-bird ; also in the 

 Gigantic Crane, and Storks in general. In 

 the Humming-birds, where the sternum is so 

 disproportionately developed, the furculum ter- 

 minates almost opposite the commencement of 

 the keel, but at some distance before it; in 



* Mr. Vigors has noticed the absence of the os 

 furcatorium in Psittacus mitratus, Platycercus eximius, 

 and Psittucula Galgula. 



those species in which we have examined it, be- 

 longing to the genus Trochilus, Lacrp. it is of 

 equal length with the coracoideum, and not 

 shorter, as Meckel asserts. As the principal 

 use of this elastic bony arch is to oppose the 

 forces which tend to press the humeri inwards 

 towards the mesial plane, during the downward 

 stroke of the wing, and restore them to their 

 former position, the clavicles composing it are 

 stronger, and the angle of their union is 

 more open, as the powers of flight are enjoyed 

 in greater perfection; of this adjustment the 

 Swifts, Goat-suckers, and Diurnal Birds of 

 Prey afford the best examples. 



Notwithstanding the anterior extremity is 

 limited to one function, and the motions of its 

 parts are confined to simple folding and exten- 

 sion, it contains the same number of joints as 

 the arm of the Monkey, or of Man himself. We 

 shall now successively consider the bones of the 

 Brachium, Antibrachium, Carpus, Metacarpus, 

 and Digits. 



The brachium, or humerus (w, Jig. 125, 

 m, fg. 130) is principally characterized by 

 the forms of its extremities. The head, or 

 proximal extremity, is transversely oblong to 

 play in the articular cavity formed by the union 

 of the scapula and coracoid bone. It is further 

 enlarged by two lateral crests: of these the 

 superior, or external, which is angular, with the 

 thin margin turned forward, affords an adequate 

 attachment to the great pectoral muscle: the 

 opposite process has its margin rounded and 

 curved backwards, and it is beneath the arch 

 thus formed that the orifices are situated, by 

 which the air penetrates to the cavity of the 

 bone. There is always a deep depression at 

 this part, even in birds which have no air in 

 the humerus, as in the Penguins and Ostrich. 

 The distal end of the humerus is not less cha- 

 racteristic of the bird, and different from that 

 of other vertebrate animals. The articular hinge 

 is divided into two parts, one internal, which is 

 the largest, for the ulna, of an almost spherical 

 form, and one external, for the radius, of an 

 elongated figure, extending for some distance 

 along the anterior surface of the humerus. 

 The radius is thus made to describe in the act 

 of bending a greater portion of a circle than 

 the ulna, and the whole fore-arm moves in a 

 plane which is not perpendicular to the anterior 

 surface of the humerus. 



The humerus is not always developed in 

 length in proportion to the powers of flight; 

 for although it is shortest in the Struthious 

 Birds and Penguins, it is also very short in the 

 Swifts and Humming-birds. In the latter, how- 

 ever, it is characterized by its thickness and 

 strength, the size of its muscular processes, 

 and the consequent transverse extension of its 

 extremities ; while in the Cursores it is as 

 attenuated as it is short, and in the Penguins 

 is reduced to a mere lamina of bone resembling 

 the corresponding part in the paddle of the 

 turtle. In the Rasores it rarely equals half 

 the length of the body ; in most other birds it 

 is about two-thirds that length; it attains its 

 greatest length in the Albatross. In this and 

 other sea-birds, as the Gulls, Awks, and Petrels, 



