The Vertebrate Skeleton

by Sidney H. Reynolds

The inner arcade is formed by the pterygoid and palatine. The pterygoid (fig. 60, 21) is a short flattened bone, which articulates behind with the quadrate, and on its inner side with a large flattened surface borne by the rostrum, in front it meets the palatine, or sometimes ends freely with a long antero-dorsally directed point.

The palatine (fig. 60, 20) is a slender irregular bone flattened dorso-ventrally at its anterior end where it articulates with the beak, and laterally behind. It gives off at its posterior end a process, which is sometimes united with the vomer, sometimes projects forwards, and meets its fellow dorsal to the vomer. In the large space between it and the vomer is the opening of the posterior nares.

The premaxillae (figs. 59 and 60, 2) are very large, and form nearly a third of the big shovel-shaped beak. They constitute the inner, and part of the front boundary of the anterior nares, and send back a pair of nasal processes which partially separate the nasals from one another.

The outer arcade forms the slender suborbital bar, and consists mainly of two rod-like bones, which in the adult are completely fused together. The posterior of these is the quadratojugal (figs. 59 and 60, 11) which articulates with the quadrate, the anterior is the small and slender jugal or malar (figs. 59 and 60, 10). The extreme anterior part of the bar is formed by the maxillae. The main part of the maxillae however lies anterior to the suborbital bar, and extends forwards along the side of the premaxillae forming all the lateral part of the beak (figs. 59 and 60, 1); it also sends inwards a plate, the maxillo-palatine (fig. 60, A, 29), which completely fuses with its fellow in the middle line, and forms the posterior boundary of the anterior palatine foramen. The term desmognathous describes the condition of the skull in which the maxillo-palatines fuse with one another in the middle line in this way.

The quadrate (fig. 60, 12), which unites the two arcades behind, is a stout irregular four-cornered bone forming the suspensorium. It articulates by its dorso-posterior corner with the squamosal, and by its antero-internal corner with the pterygoid. The middle of its ventral surface forms a hemispherical knob with which the mandible articulates, while its dorso-anterior border is drawn out into a long point which extends towards the interorbital septum.

(3) The Mandible.

The mandible or lower jaw consists of two rami which are flattened and fused together in the middle line in front, while behind they diverge from one another and articulate with the quadrates.

Each ramus is composed of five bones fused together, one being a cartilage bone, and the other four membrane bones. The articular is the only cartilage bone of the mandible, it bears the double condyle (figs. 59 and 60, 13) or concave articular surface for the quadrate, and is drawn out behind into a large hooked posterior articular process. The articular is also drawn out into a prominent process on each side of the articular surface for the quadrate, and is marked by a deep pit opening posteriorly. The articular is continuous in front with Meckel's cartilage which forms the original cartilaginous bar of the lower jaw, and is ensheathed by the membrane bones. Of these the supra-angular forms the upper part of the mandible in front of the articular, its dorsal surface is drawn out into a small coronoid process, its outer surface also bearing a prominent process. The angular is a small bone which underlies the articular and supra-angular on the inner side of the jaw. The dentary (fig. 59, 15) forms the anterior half of each ramus, and is the largest bone of the mandible; it is fused with its fellow at the symphysis in front, and extends back below the supra-angular. The splenial is a small bone lying along the middle half of the inner side of each ramus of the mandible.

(4) The Hyoid.

With the hyoid apparatus is included the columella. This forms a minute rod of bone, one end of which is expanded and fits into the fenestra ovalis, while the other end, terminated by a triradiate piece of cartilage, is attached to the tympanic membrane. The structure is as a whole homologous with the auditory ossicles of mammals and the hyomandibular of fish.