ANATOMY OF BIRDS 



227 



develops at what will be the end of the upper beak a p-enasal car- 

 tilage, pn, to become the axis of the beak. The mouth is become 

 already better formed, the axis of its cavity pointing more forward 

 than downward ; and great changes are undergoing in parts of the 

 ear at the back corner of the mouth. The quadrate and Meckelian 

 cartilages are assuming much of their true form. The quadrate 

 develops an orbital process which extends free into the orbit, and an 

 otic process which articulates with the auditory sac and parts of the 

 exoccipital cartilage. The rela- 

 tions at this stage have not been 

 made out in the fowl, but are 

 figured and described from the 

 corresponding stage of the Eu- 

 ropean house-martin (Ohelidon 

 urbica). In Fig. 67, mk is the 

 cut stump of the Meckelian carti- 

 lage, of which ar is the articula,r 

 part ; q is the quadrate, of which 

 a backward process is seen arti- 

 culating with teo, the tympanic 

 wing of the exoccipital. Just 

 below and behind this otic pro- 



« , 1 1 . .1 Fig. 67. — The postoral arches of the house 



cess 01 the quadrate, exactly martin, at middle of period of incubation, la- 

 teral view, X 14 diameters, mk, stump of Mec- 

 Icelian or mandibular rod, its articular part, ar, 

 already shapen ; q, quadrate bone, or suspen- 

 sorium of lower jaw, with a free anterior orbital 

 process and long posterior otic process articu- 

 lating with the ear-capsule, of which teo, tym- 

 panic wing of occipital, is a part ; mst, est, sst, 

 ist, stJl, parts of the suspensorium of the third 

 postoral arch, not completed to chy ; itist, 

 tion of cartilage, the handle of mediostapedlal, to come away from teo, bring- 



which is continuous with the 



where in riper embryos is the 

 fenestra ovalis ' in which is fitted 

 the foot of the stapes or stirrup- 

 bone of the middle ear, there 

 appears a trowel-shaped projec- 



ing a piece with it, the true stapes or columella 

 av/ris ; the oval base of the stapes fitting into 



sahstance of the ear-capsule ; the *^| ^^^htSLTS: Tup^s^edM • "S," 



sickle-shaped piece behind which extrastapedlal ; ist, Infrastapedlal, which will 



. ^, , '^ ■^. . . , , unite with sth, the stylohyal ; chy and Wiy, 



IS the tympanic wing 01 the ex- ceratohyal and baslhyal, distal parts of the 



n/.niT.;i-n1 //„„\ Til,* i 1 „J? same arch; 66r, Sri, ftr 2, basibranchial, cpi- 



OCClpitai {teo). ihlS trowel Ot branchial, and ceratobranohial pieces of the 



cartilage is the upperanteriorseg- ^^„i,^e^1^tonTuT°('^tefpaAer.f "^^ ''"'"' 

 ment of the hyoidean (second post- 

 oral) arch, being to that arch what the pterygo-palatine bar is to the 

 mandibular (first postoral) arch. Several parts of this stapedial 

 cartilage are recognised, as named in the fine print under the figure. 

 If the connections of the second postoral arch were completed, as 

 those of the first are, the tongue bone would be slung to the skull 

 as the lower jaw is ; but they are not, the tract represented by the 

 dot-line from the stylohyal, sth, to the ceratohyal, chy, being, like ist, 

 above sth, only soft connective tissue. This defect of connection is 

 made up for by the great development of the hyoidean parts of the 



