32 ■ MECHANISM OF THE WING. 



clpal (the third) finger; ab, arm; BC, forearm; CD, pinion, or hand, composed of c, carpus, 

 thence to B, metacmjms or hand proper, except the bone i, this, and ed, being digits or fin- 

 gers, a, shaft of humerus; b, ulna; c, radius; d, scapholunar bone; e, cuneiform bone; 

 these last two composing wrist or carpus proper. Now the figure (1) marks two lines 

 that run to the two ends of the humerus, designating a sort of cap on either end of that bone ; 

 this cap is an epiphysis;* both ends of ulna and radius show similar epiphyses, connected 

 in the figure, as in case of the humerus, with the shaft by ivaved lines. Then, of the meta- 

 carpus, g and / are the epiphyses of, respectively, the two principal metacarpal bones 7c, the 

 third, and I, the fourth; k and I have not yet coalesced together, but lie simply opposed to 

 each other, whereas their epiphyses themselves, g and /, are seen nearly fused together. 

 h, which seems to be the epiphysis of i, is not; it is a metacarpal itself (the second), 

 bearing the digit, i; it is nearly soldered with g, in which its epiphysis is already ab- 

 sorbed. Later in life, k sends a plate-like process towards I; I and k grow together; h 

 grows into k and g ; f and g grow into Ik, with the compound result fghlk, forming a single 

 bone, THE METACAiiPjii, bearing tlie "thumb" phalanx i and the two finger phalanges m, n, 

 all three of which remain permanently separate. (Observe, that k is called the thied meta- 

 carpal, because it represents that bone in the hand of man and beasts ; that in actual 

 position it is second, h being first and I third; that ordinary birds have no first and no fifth 

 metacarpals; and that the bone i, though called "thumb," corresponds to the first joint of 

 our forefinger,) d', first finger, or thumb, the seat of the bastard wing-feathers (alula, § 58) ; 

 d", actually the second finger, but morphologically the third finger, composed of two movable 

 bones m, n. a', seat of primaries (upon whole pinion) ; 6', seat of secondaries (upon fore- 

 arm) ; c', seat of tertiaries (about and above elbow) ; a", seat of scapularies (upon pteryla 

 hiimeralis)'. This wing is shown half-spread; in closing or folding, c approaches a, and 

 D approaches B; all nearly in the plane of the paper; and in unfolding, the elbow-joint 

 B is such a perfect hinge that c cannot sink down below the level of the paper, and c is 

 similarly so hinged that d cannot fly up from the same level, as the air, pressing upon 

 the quill feathers a' and b', would tend to make It do. Observe also ; b and c are two 

 rods connecting b and o, and the construction of their jointing at B and c, and of their 

 jointings with each other at their ends, is such, that they can slide along each other a little 

 way. Now when the point c, revolving about b, approaches a in the arc of a circle, the 

 rod c pushes on towards d, /, g, etc., while the rod 6 pulls back e, I, etc. ; so that the point 

 D is brought nearer b. Conversely, in opening the wing, when c recedes from a, c pulls 

 back, and b pushes on, effecting recedence of D from b. So the angle abo cannot be in- 

 creased or diminished without similarly increasing or diminishing the angle bcd. In other 

 words, you cannot open or shut one part of the wing, without opening or shutting the other ; 

 it is like killing two birds with one stone, this wonderful bony mechanism for economizing 

 muscular power, t 



"We are now ready to examine the 



§ 57. Wing-Feathees. These all grow upon the pteryla alaris (§ 9, 6, 

 and PL i, fig. 4, s ). They are of two main sorts ; the remiges (L. remex, a 

 rower) or long quills collectively, and the coverts, tectrices (L. tectrix, 

 arbitrary feminine corruption of lector, a coverer) ; to which may be added 

 as a third distinct group the bastard quills {alula, or ala spuria). The 



§ 58. Alula (L. diminutive of ala, a wing, PI. i, fig. 1, al), or little 

 wing, is simi^Iy the bunch of feathers that grow upon the "thumb." Highly 



♦Epiphysis (Gr. epi, upon, ij/msis, gi-owth). Young bones are wliolly cartilaginous, or gristly; they harden 

 at length by deposition in the cartilage of boue-earth. This deposit begins at certain points called ossiflc cen- 

 tres. Nowin what are called "long" bones, that is, bones lilie a humerus, etc., there may be one such centre 

 for the shaft and one upon each end of the bone. The shaft ossilies first; the ends later ;"and before the bone 

 has completed its growth these ends remain distinct from the shaft with which they afterwards solder. These 

 cartilaginous or gristly caps on the ends are called epiphyses. 



t See Bekgmann, Arch./. Anat., 1839,200; COUES, Amer. Nat. v, 1870, 513. 



