394 C. J. Sundevall on the Wings of Birds. 
and the former the strongest. The carpus has only two very 
small, free hones (v and w) ; the others appear to have 
either entirely disappeared or become coalesced with the 
following part (at x). The large os metacarpi ( c ) belonging 
to the only finger developed has at its base a large tubercle 
(x), which seems to indicate coalescence with a part of the 
carpus and the metacarpus of the thumb. In some birds 
this tubercle is produced into a spine (spur) covered with 
horn; and it always bears on the anterior (radial) side the 
small thumb (i d ), composed only of a single joint. Along the 
posterior (ulnar) side of the bone lies an os metacarpi (be¬ 
tween f and v) of a third finger; but this is amalgamated 
with the former at the two ends, and only bears at its apex 
a small phalange (/) concealed under the skin. The middle 
finger consists of two joints (y and z ). 
At the base (h) the humerus is more or less enclosed be¬ 
tween the muscles, so that, although it does not project 
entirely, as in man, beyond the curvature of the surface 
of the trunk, it always does so partially (§-J). The cubitus 
and the hand, which bear the true wing-feathers, are strongly 
compressed and flattened in consequence of the form of the 
bones, as well as of the position of the muscles and structure 
of the skin. 
In the angle behind the humerus the muscles and skin, as 
in man, form two more or less distinct folds, which bound 
the axilla (fig. 2, i) ; the anterior fold is formed by the pec¬ 
toral muscles, the posterior one by the back and the margin 
of the scapular muscles. 
In the anterior angle, between the humerus and cubitus 
the skin does not lie, as usual, closely upon the muscles and 
bones, but it forms there a large fold (e), the antebrachial 
fold (plica antebrachialis). When the wing is folded up, 
this cutaneous fold would hang loosely down like a bag, if it 
were not drawn together by a very remarkably constructed 
elastic sinew, which issues from a muscle in the shoulder 
(in front of h, fig. 6) beside the m. deltoideus, and runs 
within the whole anterior margin of the fold as far as the 
carpus, where it spreads out and terminates in the skin. 
