276 



AVES. 



wedged, as it were, between their upper ends, is 

 articulated in general by an anchylosis to them 

 and the ethmoid bone. This union, however, 

 always allows of a certain elastic or yielding 

 motion to pressure from below. In the Parrots, 

 where the upper mandible is an important in- 

 strument in their climbing habits, the nasal plate 

 of the intermaxillary bone is joined to the era- 

 mum by a ligamentous substance (II, Jig. 128). 

 The two lateral or mandibular processes (r, r, 

 Jig. 127) of the intermaxillary bones diverge and 

 extend backwards, external and superior to the 

 superior maxillary bones ; and, in the Ostrich, 

 they articulate with the anterior extremities of 

 the malar or zygomatic bones. Throughout 

 their whole course the mandibular processes 

 are in close contact with, and soon become an- 

 chylosed to the superior maxillary bones. The 

 ossification of the intermaxillary bone obeys the 

 ordinary law of centripetal development. The 

 lateral moieties are still separate in the chick 

 at the conclusion of incubation; and in the 

 duckling they do not anchylose until six weeks 

 after that period. The union commences at 

 the anterior extremity, while at the opposite 

 or cranial end of the nasal process, traces of the 

 original separation may frequently be observed 

 in the full-grown bird ; these are very con- 

 spicuous in the Gulls, (Laridee). 



The superior maxillary bones (s, s, Jig. 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 Yrom 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, Jig. 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 

 nbro-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 t fig- 125, e, fig. 126, 128,) is never anchy- 

 losed with the o'ther 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, arid 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. 



Having 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. 



