. The structure and life of birds . ry where they have not much ground towander over, Gulls will remain perched for some time,though apparently uncomfortable, on the thin barallotted them. This is no proof, however, that theycould sleep upon a perch. Others, which are notordinarily perchers, are quite capable of adoptingarboreal habits. The annual flooding of great tractsof country in Siberia has brought this about in thecase of the 1 Vide Seebohms Siberia in Europe, p. 147 VI FORM AND FUNCTION 169 There is connected with this subject another strangephenomenon. In many birds a thin mus
. The structure and life of birds . ry where they have not much ground towander over, Gulls will remain perched for some time,though apparently uncomfortable, on the thin barallotted them. This is no proof, however, that theycould sleep upon a perch. Others, which are notordinarily perchers, are quite capable of adoptingarboreal habits. The annual flooding of great tractsof country in Siberia has brought this about in thecase of the 1 Vide Seebohms Siberia in Europe, p. 147 VI FORM AND FUNCTION 169 There is connected with this subject another strangephenomenon. In many birds a thin muscle, calledAmbicns, arises from the Pelvis just under the thigh-joint and passes forward on the inner side of the legto the knee, before reaching which it becomes a tendon :it curves round the knee in a little tendinous tunneloccupied by itself alone, then doubles back on theoutside of the leg and passes into one of the muscleswhich bend the toes as described above. It is verycharacteristic of birds that a muscle should, by means A. Fig. 45.—Leg of chicken, the side next the , ambiens muscle ; k, knee-joint. of a long tendon, do its work at such a distance: but,curiously, it is not found in by any means all theperching birds. And, besides, this the same muscle isto be found in This must not, how-ever, be taken to prove any very close relationship withbirds. The fact that it is found in two families of birdsmay help to prove that they are closely allied, but suchevidence is less dependable when we are dealing, not 1 Refer to Appa?-eil Locomoteur des Oiseaux (M. EdmondMix), p. 443. i;o THE STRUCTURE AND LIFE OF BIRDS CHAP. with the relationship between families within the sameclass, but with the relationship between two the crocodile the muscle in question appears eithernot to connect with the toe-flexor muscles or else tobe altogether functionlcss, tor when I have bent theankle-joint o( a young American alligator, most pro-bably resembling a croco
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