Fig. 4.—DISSECTION OF HEAD OF GREEN WOODPECKER VIEWED FROM BELOW.
(After Macgillivray.)
(l) Lower Mandible; (f) Base of Tongue; (th.h., th.h.)Thyro-hyals; (s.g., s.g.) Salivary Glands; (m, m) Muscles of Neck; (œ, œ) Œsophagus; (tr) Trachea; (e.m., e.m.) Extrusor Muscles, which thrust out the Tongue; (r.m.) Retractor Muscles of Tongue wound round Trachea; (c.tr., c.tr.) Cleido-tracheal Muscles, binding Trachea to Shoulder-girdle.
Enclosed in the sheath here spoken of, and along the concavity of each bone, is a muscle which has a fixed attachment to the crura of the lower mandible on each side (Fig. 4, e.m., e.m.). The contraction of this muscle shoots the tongue out in two different ways. In the Green Woodpecker the extremities of the thyro-hyal bones are themselves attached to the mandible, while the curvature of the bones makes a loop that hangs low down on each side of the neck (see Fig. 2, th.h.). As the muscle is shortened this loop is raised up, and the free tip of the tongue is consequently projected; and since the muscle is on the inner, or concave, side of the curve, a very small shortening on its part makes a great addition to the apparent length of the tongue. Sir Charles Bell elucidates this action by comparing the great effect on the curve of a fishing-rod’s flexible top that a small tightening of the line has. But while this is the case in many species, there are others in which the sheath alone is attached to the bones of the forehead, and the bones themselves slide along inside together with the contracting fibres of the muscle, thus producing the same result as was obtained in the other case by the loops hanging low down in the neck.
The tongue, whose length is thus so extraordinarily increased, is drawn back to its original position within the bill by another pair of muscles, one on each side, which are attached to the basi-hyal. These take their origin from the trachea, around which (as shown in Figs. 2 and 4, r.m.), in many species, they are curiously wound in their course. And, since the bones are at the point of their greatest curvature when at rest, it is obvious that this action of withdrawal is materially assisted by the elasticity of the prolongations of the hyoid bones themselves; for it is a well-known law that Nature never lets power run to waste, but always utilises forces of mere elasticity or rigidity when by their means the expenditure of nervous energy and muscular contractility can be saved.
WRYNECK.
It may be observed that this curious development of the bones of the tongue is not confined to the Woodpeckers; in the Sun Birds (Nectariniidæ) of the Old World, and the Humming-Birds (Trochilidæ) of the New, this same adaptation of means to ends obtains. Even in the Picidæ themselves many variations have been noticed, in addition to those above alluded to; for instance, in the Yellow-billed Woodpecker (Sphyrapicus[255] varius) of North America the horns of the hyoid do not reach so far as the eye, so that the tongue, with its bushy tip in this case, is only extensible in a very slight degree; while in the Hairy Woodpecker (Picus villosus) the thyro-hyals curve spirally over the right orbit so as to reach entirely around the eye, to be inserted at its lower posterior margin.
GREAT BLACK WOODPECKER AND GREAT SPOTTED WOODPECKER.