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Show 1876.] ANATOMY OF PLOTUS ANHINGA. 339 associated contracts. In fact, a pulley has to be formed round which the tendon may turn in the same manner that at the knee, in birds, the biceps cruris is able to act upon the fibula from a point situated some way down it, because it is bound close to the greatly bent knee-joint by the well-known sling-band in that region. In Plate X X V I . figs. 1 & 2, the sling-band here described is clearly shown. A similar sling-band is found in the posterior cervical region of those birds which have any great backward curve of the neck, it in the Gannets being also associated with the 9th vertebra. It is nothing more than a specialization and strengthening of the aponeurosis which is always found covering the muscles, opposite the place where the strain occurs. In Phalacrocorax carbo the general sheath is strong, and no specialized band can be distinguished. In Plotus anhinga this sling-band is attached at its inner end, with its fellow of the opposite side, to the middle line of the posterior surface of the neural arch of the 9th vertebra, about halfway between its proximal and distal extremities ; whilst at its outer end it is fixed to the tubercle which is situated just outside the upper articular process of the same vertebra, a backward and slanting loop of tendinous tissue joining the two (vide Plate X X V I . figs. 1 & 2). It is the ossification of this just-described tendinous loop which constitutes Donitz's bridge in P. levaillantii and P. novce-hoillandia. In neither of the Society's specimens of P. anhinga, nor in that figured by Brandt, nor in a specimen of the same species seen by Donitz himself, is this bridge ossified. It m a y therefore be that in P. anhinga it remains tendinous ; or, less probably, it has happened that the four specimens inspected have none of them been adult birds. One is at least 3^ years old, we know. Through this bridge, as has been briefly described by Donitz, from a dried specimen, the tendon of the longus colli posterior passes - which it does before it receives the considerable fleshy fasciculus originating from the neural arch of the ninth vertebra, as is rendered evident in Plate X X V I . figs. 1 & 2. The tendon, augmented by fibres from the just-mentioned additional origin, passes up the back of the neck, side by side with its fellow, to end by being inserted into the posterior surface of the lower articular process of the axis vertebra, it, in its course, sending small tendinous slips to the corresponding parts of the third and fourth cervical vertebrae. The tendon is peculiar in being ossified where it is opposite the bodies of the vertebrae with which it is related, and supple at the joints, which makes it appear to be composed of alternate bony and fibrous elements when it is removed from the body. From the fifth, fourth, and third vertebrae short muscular fibres ascend to join the corresponding portions of the main tendon, in single penniform series. It is nearly always the case in avian anatomy that the inner fibres of the cervical portion of the longus colli posterior muscle become differentiated to form the digastrique du cou of Cuvier, better known to us as the biventer cervicis, a muscle one peculiarly interesting 22* |