SEC. IV ANA TOMY OF BIRDS 239 



The Quadrate Bone (Lat. quadratus, squared ; Figs. 62, 63, n, 

 64, 65, 68, 69, 71, q, 75, Qii), with which we may begin the jaw- 

 bones proper, is the suspensorium of the lower jaw, — the perfectly 

 constant and characteristic bone by means of which the mandible 

 proper articulates with the skull. Its rudiment is seen in the 

 earliest embryos, at the corners of the primordial parachordal 

 cartilages. It belongs to the mandibular (first postoral) arch, of 

 which it is the proximal element. Its general morphology has 

 caused much dispute. From the fact that in birds one of its func- 

 tions is to supi)ort, in part, the tympanum of the ear, it has been 

 identified with the tympanic bone of mammals, — that which in man 

 forms the bony tube of the external auditory meatus. The view 

 now generally accepted is, that the bird's quadrate represents, cer- 

 tainly in part, probably in whole, the little bone of the middle ear 

 called the malleus in mammals. However this may be, the quadrate 

 of a bird bears the proximal ends of both jaws, carrying their final 

 (posterior) articulation up to the squamosal and petrosal bones. 

 Thus, the foot of the quadrate forms the free hinge of the lower 

 jaw, and also movably articulates the back end of both the zygo- 

 matic and the pterygopalatine bars or "arcades." The head of 

 the quadrate freely articulates with the squamosal, just in front of 

 the tympanic cavity, which it thus bounds in front ; and there is 

 usually a shoulder which furthermore articulates with the anterior 

 periotic bone, the prootic. Struthious birds do not have these two 

 distinct facets. A long pedicle or orbital process extends forward, 

 inward, and upward in the orbit ; this non-articular handle is for 

 advantageous muscular traction. So circumstanced, the quadrate 

 is a stocky bone, of a shape reminding one of an anvil ; it rocks 

 freely to and fro upon its cranial socket, pulling and pushing upon 

 the Avhole maxillary and mandibular mechanism, with such eftect 

 that when the lower jaw drops, the zygomatic and palatal bars are 

 automatically shoved forward, tending to make the upper jaw rise, 

 and so increase the opening of the mouth. Such mobility of the 

 upper jaw automatically with the movement of the lower is very 

 free in parrots, whose craniofacial connections are quite articular in 

 character ; it is well shown also in ducks ; and probably nearly all 

 birds have some such motion of the upj^er jaw upon the skull. In 

 nearly all birds, tlie mandibular articular facet of the quadrate is 

 divided by a lengthwise impression into inner and outer protuber- 

 ances, or condyles, fitting corresponding depressions on the articular 

 face of the lower jaw ; in some birds the articular surface is single. 

 The zygomatic articulation with the quadrate is made by the balled 

 end of the quadratojugal socketed in a cup at the outer side of the 

 mandibular facet (with various minor modifications in different 

 birds). The palatal articulation is made by a little condyle of the 



