The superior maxillary bones (s, s, fig.126, 127) are very seldom united together in birds. They are comparatively of small size. Each may be said to commence mesiad of the ori gin of the mandibular processes of the in termaxillary bone ; it then expands as it pro ceeds backwards, and, opposite the anterior end of the palatine bone, divides into two processes. The mesial or palatine process ex tends along the outside of the palatine bone, and soon becomes anchylosed to it; the ex ternal or malar process is articulated obliquely to the under part of the anterior moiety of the zygomatic bone. At the origin of this process a small projection meets the descending pro-. cess of the nasal bone. In most Gallinaceous birds, the body, or part anterior to ,the palatine and zygomatic processes, is wanting.; but in the common fowl it extends towards the mesial line, and unites with the vomer, so as to divide the palatal fissure into an anterior and posterior cavity. In the Ostrich, where the body of the upper maxillary extends for wards to the symphysis of the intermaxillary bone, a process is also given off at the origins of the palatine and zygomatic bones, which passes inwards to the vomer, and completes, in the adult, the boundary of the anterior pa latal fissure.
The movement of the bony framework of the upper mandible resulting from the union of the intermaxillary, superior maxillary, and palatal bones, is immediately effected by the elongated malar or zygomatic bone, (o, fig. 125, t, t, fig. 126, 127, 128,) which transfers to the zygomatic process of the superior maxil lary the movements of the tympanic bone, being so placed as to form the medium of communication between these parts. It ex tends in a straight line from one to the other, this being the form best adapted to resist the pressure upon its two extremities. With the superior maxillary bone it is soon anchylosed, but with the tympanic bone it is in most Birds articulated by a moveable ball and socket-joint, the articular surfaces being connected by a fibro-cartilaginous substance ; in the Capri mulgi, however, it is anchylosed at both ex tremities. The malar bone is commonly of a compressed or vertically flattened form, but sometimes, as in the Ostrich, it is cylindrical. It is originally composed of two pieces placed in a parallel line, one above the other; the superior being pointed at both extremities, and much smaller than the other.
The tympanic, pedicellate, or quadrate bone (i, fig. 125, e, .fig. 126, 128,) is never ancliy losed with the other elements of the temporal bone, but is freely moveable as in most of the cold-blooded ovipara; and it is interesting to observe that in the rodent quadrupeds, which exhibit many other affinities to birds, the tym panic element remains for a long period a de tached bone, but is situated altogether posterior to the maxillary articulation. In birds, where
the base of the cranium is remarkably shortened in the antero-posterior diameter, the tympanic bone is, as it were, thrust forward and wedged in between the inferior maxillary bone and the zygomatic process of the temporal, thus inter cepting, and articulating with, both the lower jaw and cheek-bones. The membrana tympani continues, however, to be attached by about half its circumference to the posterior part of the os quadratum, and for the remainder of its extent to the occipital and sphenoidal bones.
The upper end of the tympanic bone is articulated by two distinct transverse condyles with the zygomatic portion of the temporal bone; below these it is contracted, and then expands as it descends, giving off a strong process from the middle of its anterior surface, which projects into the orbit, then a smaller process from its posterior surface extending backwards, and lastly, sending off at its lower extremity an external process for the malar bone, and an internal one for the pterygoid, between which processes are two oblique oblong convexities for the articulation of the lower jaw.
lIavin,g an immediate connection with the mo tions of the whole beak, it necessarily presents varieties of form in different birds, without, however, losing the characteristic figure which has been described. By whatever cause the tympanic bone is carried forwards, whether by the action of the pterygoid muscles in serted into its orbitar process, or by the pres sure of the lower jaw upon its inferior surface, that motion is communicated to the pterygoid and malar bones, which transfer it, the one to the palatine, the other to the superior max illary bones, and thus the upper mandible is elevated at the same time that the lower one is depressed. The elasticity of the union of the nasal process of the intermaxillary bone with the cranium restores the upper jaw on the cessation of the pressure from below, to the position from which the move ment of the tympanic bones had displaced it.
These movements are freely allowed in most birds from the nature of the articulation of the tympanic bone; but in the Struthious birds they are more restrained, from the connection of the bone with the descending zygomatic process of the temporal bone ; the extent of this attachment is greatest in the Emeu, where it almost produces a complete fixation of the tympanic bone.