STRUCTURE OF BIRDS. 29 



feathers, variously proportioned, according to the kind of flight 

 required by the species ; and as they are not used for supporting 

 it on the ground, as in quadrupeds, or for enabling it to cling 

 to an inclined surface, the claws are obliterated, and even the 

 fingers so modified as to be scarcely distinguishable. The 

 coracoid bone, /^, resembling the clavicle in man and quadru- 

 peds so much as generally to have been mistaken for it, is on 

 the contrary considered analogous to the coracoid process of the 

 scapula in the mammalia, while the furcular bone, j^ is taken 

 for the acromial process. These parts, however, being very 

 remarkably modified in birds, I shall speak of them as the 

 coracoid bones, /^, h ; the scapulas, z, i ; and the furcul^E, j^ j. 



From the anterior margin of the sternum, in an elongated 

 fossa of which they are articulated, proceed obliquely outwards 

 the two coracoid bones, or posterior clavicles, /^, 7^, which are 

 the strongest of those forming the shoulder, and aftbrd the chief 

 support to the wing. Near its anterior extremity, on the outer 

 side, is a small hollow, /, which, with a corresponding one on 

 the scapula, m^ forms the glenoid cavity, for the reception of 

 the head of the humerus. 



The scapula, ?', /, is a narrovv^ elongated bone, flattened to- 

 wards its posterior extremity, generally curved, and lying over 

 the ribs. At its anterior end it is articulated to the coracoid 

 bone behind the glenoid cavity, ?z, and by a process to the fur- 

 cula, 0. 



The furcula, or anterior clavicle, J, y, J, varies extremely in the 

 different families of birds. It commonly presents the appear- 

 ance of a single bone, bent so as to resemble a V or U ; but in 

 some few cases its two sides are not united below. Sometimes 

 its angle is anchylosed to the sternum, but more commonly 

 connected by a membrane with it. The use of this bone being 

 to prevent the humeri from being driven inwards by the action 

 of the pectoral muscles in flying, it is stronger, and has a wider 

 arch, in those species which have the most powerful flight. 



Turning again to Plate I, where the coracoid bone is marked 

 r ; the furcula, 5, s ; and the scapula, t ; we can trace the other 

 bones of the wing, which resemble those of the human arm. 



The OS humeri, or arm-bone, «, u^ is of a cylindrical form, 



