The inferior maxillary bone (p, fig. 125, v, figs. 126, 128,) is originally composed of twelve distinct pieces, each lateral moiety being made up of six. The anterior symphyseal or dental portion of each ramus first unites with its fellow at the symphysis ; the two portions which form the condyle (v, fig. 126) next an ehylose ; the angular (v, fig. 126), supra angular and opercular, or splenial pieces are consolidated at a later period. The anterior extremities of the angular and supra-angular pieces are wedged into corresponding grooves of the symphyseal element ; and the opercular portion is extended like a splint along the inner side of the gomphosis, by which the preceding portions are united.
The traces of the original separation of these bones long remain in the semi-aquatic and aqua tic birds ( Grallatores and Natatores), which, as the lowest of the class, manifest their affinity in this respect to the cold-blooded Ovipara, where this complex structure of the lower jaw continues throughout life.
As the lower jaw, thus constituted, forms with the upper jaw the principal organ of pre hension in birds, it presents many variations of form and magnitude, which immediately relate to, and are consequently indicative of their mode of life, food, &c. These general modifications will be treated of in relation to the digestive function, but some of the less conspicuous characters of the lower jaw may be more appropriately considered in this place.
The rami are in general completely anchylosed at the symphysis, the extent of the united por tions varying considerably in different birds, but occupying in most cases only a small pro portion of the jaw. In the Pelicans the rami are united by the mere extremities, appearing as if bent upon each other at the symphysis, and supporting the dilatable sac which fills up the intermediate space, like the hoop of the fisher man's landing-net. The symphysis is also of very small extent in most other Palmipeds. It is small in the Rasores and Cursores. In the Storks and Cranes it extends along a third part of the entire jaw. In the Flamingo, where the anterior part of the jaw is bent down at an obtuse angle, nearly half of the rami are united. In the Skimmers (Rhyncops), Horn bills, and Toucans, two-thirds. In the Curlew the two rami are in apposition for two-thirds of their anterior extent, but are not anchy losed, and form, in this respect, the only known exception to the rule.
In diurnal Birds of Prey, in many of the Parrot-tribe, in the Herons and Swans, each ramus of the lower jaw forms an entire bony plate. In the rest. of the class a membranous unossified space is left at the place of union of the symphyseal with the angular, supra angular, and splenial elements. This defi ciency is of a longitudinal form, and is always situated behind the middle of the ramus. In the Bustards, Woodcocks, Curlews, Gulls, Skimmers, Guillemots, Petrels, and Pen guins, there is a second foramen, of a rounder figure, posterior to the preceding, and resulting from a defective union of the angular, supra: angular, and condyloid pieces. In the Casso wary this space is subdivided into several small foramina. In the Emeu ( Dromaius) and Ostrich (Struthio) there is a single small fora men at the corresponding part.
At the posterior part of each ramus the fol lowing processes are developed in various degrees in different birds. The suprangular piece ascends in a greater or less degree in the form of a thin lamina with a gently rounded outline, representing the coronoid process. From the inner side of the condyloid piece there extends a more marked process, which may be called the internal angular; and from the posterior part of the ramus a third process is continued, which may be termed the posterior angular process.
The coronoid process is most developed in the Parrots, Gulls, Herons, and Cross-bills (Loxia), in some of which, as the Loxia coccothraustes, cardinalis, and pulverulentus, the lower jaw presents the following peculiarity. A large sesamoid bone of a triangular form, but rounded and transverse, with the base directed outwards and the apex inwards, is situated at the posterior and internal aspect of the articular ligament of the lower jaw. It com pletes the maxillary articulation posteriorly, and corresponds by its anterior articular surface to the posterior part of the outer condyle. The ar ticular surface of the lower jaw of the Parrots is a simple narrow longitudinal furrow, open at the two extremities. That of the Toucans is almost equally simple, but of a rounder figure. In most other birds the articular surface is divided into two distinct portions, of which the internal is an oblique concavity, the exter nal also oblique, but terminating in a convex eminence behind.