Fig. 2.—SIDE VIEW OF DISSECTION OF HEAD OF COMMON GREEN WOODPECKER.
(Half natural size. After Macgillivray.)
(u, l) Upper and Lower Mandibles; (t) Barbed Tip of Tongue; (th.h.) Thyro-hyal Bone of Right Side, with its Muscle and Sheath; (o) Right Orbit; (n) Right Nostril; (s.g.) Right Salivary Gland; (m, m) Muscles of Neck; (œ) Œsophagus; (tr) Trachea; (r.m.) Rectractor Muscles of Tongue wound round Trachea.
Another woodcut (Fig. 2) shows a side view of a dissection of the head of the common Green Woodpecker (Gecinus viridis), and a reference to the explanation of the lettering on it will give a general idea of the whole.
The tip of the tongue (t) is a slender, flattened, horny point, bearing on its sides and upper surface a number of very delicate bristles, or prickles, directed backwards, an arrangement eminently useful to the bird for enabling it to extract its insect food from the recesses to which its beak, by reason of its size and hardness, could not readily, nor with sufficient quickness, gain access. This tip is further rendered a more efficient instrument for this purpose by its being constantly moistened by a very viscid saliva secreted by two particularly large salivary glands (Figs. 2, 3, and 4, s.g.); and it was long ago remarked by Sir Charles Bell, in his essay on “The Hand” (Bridgewater Treatise, 1837), that the same muscles that effected the protrusion of the tongue exerted a simultaneous pressure upon these glands, so that the first result of the muscular contraction is to lubricate the tongue, while the rest of its force is spent in shooting it out with marvellous rapidity.
Fig. 3.—UPPER VIEW OF SKULL OF GREEN WOODPECKER.
(After Macgillivray.)
(th.h, th.h.) Thyro-hyal Bones; (i) Point of their insertion; (s.g., s.g.) Salivary Glands.
Behind this barbed and horny tip, the tongue is a slender worm-like body, of which the core is the anterior prolongation of the hyoid bone. The fore-part of this core, more like a bristle than a bone, is known to anatomists as the “glosso-hyal,” and it is immediately succeeded posteriorly by the “cerato-hyal.”[254] Behind this is the “basi-hyal ” (Fig. 1, b.h.), the last bone to enter into the formation of the tongue proper. From this basi-hyal springs the pair of bones—the “thyro-hyals”—which attain the remarkable degree of development for which the birds now under consideration are distinguished. From each side of the hinder portion, then, of this basi-hyal bone diverge these important “thyro-hyals” (Fig. 1, c.br., e.br.). They, in the Woodpeckers (compare Fig. 3, th.h.), extend outwards and backwards to pass one on each side of the neck until they curl upwards and forwards, converging to meet one another on the upper part of the back of the head; thence they run along together, ploughing themselves a furrow in the skull-top till they reach almost to the right nostril. Each of these curved and highly elastic bones is surrounded by a delicate sheath, whose inner surface is kept constantly moist and lubricated by its own secretion; and this sheath is attached to the bone of the skull at its junction with the upper mandible, as is shown in the accompanying woodcut (Fig. 3, i).
