OCR Text |
Show 212 ON THE ANATOMY OF THE INDIAN DARTER. [Feb 7, no further flexion is possible. The 8th vertebra is thus so articulated with the 7th anteriorly and the 9th posteriorly as to allow it, when the neck is flexed, to be nearly at right angles to the rest of the neck, the two portions of which, though parallel, are then at different horizons, something like the two bars of a parallel ruler (vide diagram, p. 211, fig. 1). W h e n the neck is bent in this Z-shaped form, any opening out of the anterior angular bend by the action of the anterior neck-muscles causes the anterior moiety of the neck to suddenly shoot out, thus causing a corresponding protrusion of the head and beak (diagram, fig. 2). B y the flexion of the 6th on the 7th, and of the 9th on the 10th, cervical vertebrae, the curve of the neck is increased-the articulations of the 8th vertebra still forming the double hinge round which motion takes place-and the impaling action correspondingly augmented. This protrusion, though only for a short distance, is so violent as to effectually "strike " the fish which the bird is pursuing. The bending-back of the neck is effected, partly by the action of the longus colli posterior, partly by a special pair of closely approximated muscles, situated anteriorly along the middle line of the neck, which arise close together from the haemapophysial spine of the 11th cervical vertebra, near its anterior articular end, and are inserted into the sides of the anterior half of the 6th cervical. The opening-out, on the other hand, of the genu formed by the 7th and 8th cervicals-by which, as already described, the impaling action is produced-is caused by the contraction of the tho-racically very powerful longus colli anterior. The main tendon of this is inserted on the long, backwardly-directed haemapophysis of the 8th cervical, playing round the doubly-grooved surface of the inferior arch formed by the haemapophyses of the 9th cervical, to which vertebra, as well as to the 10th, it gives off much smaller tendinous slips. It is obvious that considerable advantage is gained by the action in question, the rapid protrusion of the narrow neck and head over a small space by this mechanism necessitating a less amount of exertion than would a similar movement of the whole bird over the same space, and being equally efficacious in striking the prey. The whole mechanism, it may be observed, exists in a less developed form in the neck of the Herons, Cormorants,&c; and it requires but a slight modification of the arrangement of these parts in those birds-none of which, so far as I know, impale their prey like the Darters-to bring about the perfect adaptation of these structures to a newly acquired mode of feeding. |