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Show 1903.] SYRINX OF THE ACCIPITRES. 159 Genus POLYBORUS. The syrinx of P. brasiliensis is constituted upon the plan of Falco, but there are a number of differences in detail. There is the same extensive membrana tympaniformis externa into which is inserted the intrinsic muscles. But the last six tracheal rings are more thoroughly fused into a box, this fusion taking place in front. At the same time the last three or four of these rings increase rapidly in diameter so that the lower part of the trachea is very wide, much more so than in Falco. The intrinsic muscles, moreover, are distinctly smaller in proportion in Polyborus. Genera TINNUNCULUS and ERYTHROPUS. The syrinx of the Common Kestrel (T. alcmdarius) appears to me to need no special description. It is quite like that of Falco, as is also that of Erythrops vespertinus, which genus perhaps hardly needs to be separated from Falco. Genus NISAETUS. The syrinx of Nisaetus fasciatus (text-fig. 17) will serve as a type of the Aquiline form of this oigan as contrasted with the Falconine. Text-fig. 17. Syrinx of Nisaetus fasciatus (from a drawing by the late Prof. Garrod): the left-hand bronchus seen from in front, the right from behind. X 2. The most salient difference is the absence of a specially enlarged area of membrane between two of the early bronchial semirings ; that is to say, the membrana tympaniformis externa is not a conspicuous feature of the syrinx of this bird, as it is of that of Falco. The ring which gives rise to the pessulus in front is separated by four semirings from that which receives the insertion of the intrinsic muscles; but some of these rings, three at any rate, are |