The bones of the fore-limb and the neighbouring regions show a remarkable modification of the primitive plan. The bones of the upper arm OA, (Fig. 181) and fore-arm (Rd., Ul.,) are arranged much like the corresponding bones of the rabbit; but those of the wrist and hand are unlike anything we have yet examined. The fourth and fifth fingers have entirely disappeared; and the first (thumb), second, and third, though they can be still recognised (Figs. 181, F, F, F and 184a), have become consolidated with what remains of the wrist (HW, MH) to form a firm mass which is in no danger of “giving” during the powerful down-stroke of the wing. The head of the bone of the upper arm fits into a socket (gl.cv., Fig. 182) at the junction of the shoulder-blade (scp.) and another and stouter bone (cor.) which runs back and is attached to the breast-bone or sternum (st.). The sternum in its turn is connected by means of ribs with the spinal column, which is strengthened here by the fusion of some of its bones. The ventral part of the sternum is produced into a large plate called the keel (car., Fig. 182), which gives attachment to the great muscles of the breast, used in the movements of the wings. So great is [Pg 275] the development of these muscles in flying birds that in the pigeon they have a weight equal to one-fifth the total weight of the body. The three segments of the wing, corresponding to the upper-arm, fore-arm, and wrist and hand respectively are, in the position of rest, bent upon each other like the letter Z. The large quill feathers of the wing, fixed along the hinder border of the limb, are in two series (Fig. 179). Those attached to the fore-arm are called secondaries; while those attached to the wrist and hand are known as primaries. A smaller tuft of feathers borne by the thumb is called the bastard-wing or thumb-wing. The smaller feathers covering the bases of the wing-quills are called wing-coverts. Fig. 182.—Pigeon. The bones of the trunk. actb, socket for bone of upper leg; car, keel of sternum; cd.v, vertebrae of tail; cor, coracoid; fur, merrythought; gl.cv, socket for bone of upper arm; pyg.st, ploughshare bone; scp, shoulder blade; s.scr, fused vertebrae of hip region; st, sternum; st.r, vr.r, rib; th.v.1, first, and th.v.5, last thoracic vertebra. (× ⅓.)