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Show 1889.] ANATOMY OF POLYBOROIDES. 81 the Falconidse. Polyboroides is in these particulars near to the Falconidse. Cathartes is described and figured by Fiirbringer . Serpentarius is neither figured nor described in Fiirbringer's work. The line of attachment of the deltoid to the humerus and, in consequence, the size of that muscle vary in the three types of Accipitres. It is largest in Serpentarius and smallest in Cathartes; in the former bird the length of the attachment of the muscle to the humerus is about half the entire length of the bone. In Gathartes the proportion is (roughly) as I : 4. Gypohierax is intermediate but nearer to Serpentarius. Polyboroides agrees with Gypohierax. In all Accipitres diurnse the anconeus has an accessory head arising from the humerus close to the insertion of the latissimus dorsi. But there are some differences of detail. In Serpentarius (fig. 2) the accessory head of the anconeus forms a particularly broad flat tendon. The anterior of the two latissimi dorsi end, as in most birds, in a narrow tendon ; this gives off a branch to the integument just before its insertion. In Gypohierax the accessory head of the anconeus is very narrow, but the relations of the latissimus dorsi to it and to the integument are precisely as in Serpentarius. Cathartes is rather different from both these types ; the accessory head of anconeus is almost completely split into two, the thickness of the tendon being very unequal in different parts. The latissimus dorsi tendon splits into two as in Serpentarius and Gypohierax; one tendon passes above and the other below the posterior latissimus dorsi close to its insertion ; the uppermost of these is attached to the belly of the anconeus. Polyboroides, as in other myological relations, comes nearest to Gypohierax, but I a m unable to state whether the branch of the latissimus dorsi tendon to the integument is present. It is also worth remarking that while the scapular head of the anconeus in Cathartes is distinctly double and entirely tendinous- one tendon arising from the scapula itself, the other from the supinator muscle-this muscle originates in all the remaining types (including Polyboroides) from the scapula alone and by a single head, which is chiefly fleshy though partly tendinous. The size of the second pectoral muscle offers characters by which the affinities of Polyboroides can be to some extent determined. In Cathartes aura the attachment of that muscle reaches nearly to the end of the carina sterni. In Gypohierax angolensis the muscle reaches only for a very short distance along the carina sterni; this is also the case with Serpentarius and Polyboroides. The proportions between the total length of the carina sterni and the breadth of the second pectoral muscle where it is attached close to the base of the carina sterni are indicated in the following table:- 1 Loc. cit. pi. xxii. fig. 7. PROC. ZOOL. Soc-1889, No. VI. 6 |