154 



GENERAL OBNITHOLOGY. 



the exoccipital (feo). This trowel of cartilage is the upper anterior segment of the hyoidean 

 (second post-oral) arch, being to that arch what the pterygo-palatine bar is to the mandibular 

 (first post-oral) arch. Several parts of this stapedial cartilage are recognized, as named in the 



fine print under the figure. If the connections of the 

 second post-oral arch were completed, as those of the 

 first are, the tongue bone would be slung to the skuU 

 as the lower jaw is ; but they are not, the tract rep- 

 resented by the dot-line from the stylo-hyal, sth, to 

 the cerato-hyal, 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 third post-oral arch, hr 1 and hr 3, which 

 retain the tongue-bone in position, vi'ithout however 

 articulating it with the skuU. The hand of the trowel 

 of cartilage soon segments itself ofl' from the ear-cap- 

 sule, bringing away with it a small oval piece of the 

 periotic wall, which piece is the true stapes, and the 

 oval space in which it fits is the fenestra ovalis leading 

 into the inmost ear (the cochlea'). The broad part of 

 the trowel-blade is the extra-stapedial part, on vrhich 

 the memhrana tympami, or ear-drum, vrill be stretched. 

 The stylo-hyal, sth, wiU join the extra-stapedial 

 plate, and the afterward chondrified band of union wiU 

 be the infra- stapedial, ist. (Figs. 71, st, and 83.) 



Returning 

 now to the 

 chick's head, 

 which we left 

 to examine 

 the intricate 

 ear - parts at 

 the proximal 

 end of the second post-oral arch, we see by fig. 68 

 how rapidly the parts are shaping themselves at the 

 end of this second stage of development. This figure 

 shows the cartilaginous skull, in which no trace of 

 ossification has appeared, excepting in the under 

 mandible. The brain and membranous parts of the 

 cranium have been removed. The roof of the skull 

 never becomes • cartUaginoua, bone there growing di- 

 rectly from the membrane; and the whole of the chon- 

 dro-cranium, as shown in the figure, is one continuous 

 cartilaginous structure (like the whole skull of an 

 adult shark or skate), excepting the parts of the post- 

 oral arches, which are separate. The auditory cap- 

 sule is environed by occipital cartilage, eo, stretching 

 over the back of the skull, and by wing-like growths 

 (alisphenoids, as) which wall most of the brain-box 

 in front. The high orbito-nasal septum is a continuous vertical plate of cartOage, upgrowing 

 from the tract of the conjoined trabecules. Lateral developments of this ethmoidal wall, in 



Fig. 67. — The post-oral arches of the 

 house martin, at middle of period of incuba- 

 tion, lateral view, x 14 diameters, mk, stump 

 of meckelian or mandibular rod, its articular 

 part, ar, already shapen ; q, quadrate bone, or 

 suspensorlum of lower jaw, with a free anterior 

 orbital process and long posterior otic process 

 articulating with the ear-capsule, of which teo, 

 tympanic wing of occipital, is a part ; msi, 

 est, sst, ist, sth, parts of the suspensoriura of 

 the third post-oral arch, not completed to chy ; 

 vist, medio-stapedial, to come away from teo, 

 bringing a piece with it, the true stapes or co- 

 lumella auris ; the oval base of the stapes fit- 

 ting into the future fenestra ovalis, or oval 

 window looking into the cochlea ;sst, supra-sfio- 

 pedial ; est, extra-stapedial; ist, infra^stapedial, 

 which will unite with sth, the stylo-Iiyal ; 

 cky and bhy, cerato-hyal and basi-hyal, distal 

 parts of the same arch ; 66r, br 1, br 2, basi- 

 branchial, epi-branohial and oerato-branchial 

 pieces of the third arch, composing the rest of 

 the hyoid bone ; tg, tongue. (After Parker.) 



Fig. 68. — Skull of cMck, second stage, in 

 profile, brain and membranes removed to 

 show cartilaginous formations, X 4 diameters. 

 eth, ethmoid, forming median nose-parts and 

 inter-orbital septum ; developing lateral parts, 

 as ate, aliethmoid, als, aliseptum, aln, alinasal, 

 pp, partition between nose and eye; pn, pre- 

 nasal cartilage ; ps, presphenoidal part of mid- 

 ethmoid; 2, optic foramen; as, alisphenoid, 

 walling brain-box in front ; pf, post-frontal, 

 bounding orbit behind; joa,j?ff, palatine and 

 pterygoid; q, quadrate; so, supra-occipital; 

 eo, ex-occipital; oc, occipital condyle, borne 

 upon basi-occipital, and showing nc, remains 

 of notochord ; these occipitals bound the fora- 

 men magnum, and eo expands laterally to form 

 a tympanic wing, circumscribing the external 

 auditory orifice behind and below; hsc,psc, 

 horizontal and posterior vertical semicircular 

 canals of ear,- fi; st, fenestra rotunda and 

 fenestra ovalis, leading into inner ear, lat- 

 ter closed by foot of the stapes ; mk, ch, bh, 

 bbr, cbr, ebr, parts of jaw and tongue, as nam- 

 ed in figs. 65, 66 and 67. (After Parker.) 



