154 
the exoccipital (¢eo). 
GENERAL ORNITHOLOGY. 
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 
_ Sse 
4 est 
“eo 
Fic. 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; g, quadrate bone, or 
suspensorium 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; mst, 
est, sst, ist, sth, parts of the suspensorium of 
the third post-oral arch, not completed to chy; 
mst, 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-sta- 
pedial; est, extra-stapedial; ist, infra-stapedial, 
which will unite with sth, the stylo-hyal ; 
chy and bhy, cerato-hyal and basi-hyal, distal 
parts of the same arch; bbr, br 1, br 2, basi- 
branchial, epi-branchial and cerato-branchial 
pieces of the third arch, composing the rest of 
the hyoid bone; ¢g, tongue. (After Parker.) 
(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 skull 
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, br 1 and br 2, whieh 
retain the tongue-bone in position, without however 
articulating it with the skull. The hand of the trowel 
of cartilage soon segments itself off 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 which 
the membrana tympani, or ear-drum, will be stretched. 
The stylo-hyal, sth, will join the extra-stapedial 
plate, and the afterward chondrified band of union will 
be the infra-stapedial, ist. (Figs. 71, st, and 83.) 
Returning ps 
now to the 7 a 
chick’s head,  *”° H ne 
which we left 
to examine 
the intricate 
ear-parts at 
the proximal Fic. 68. — Skull of chick, second stage. in 
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 cartilaginous, 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. 
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 ale, aliethmoid, a/s, aliseptum, alm, 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; pa, pg, palatine and 
pterygoid; g, quadrate; so, supra-occipital; 
eo, ex-occipital; oc, occipital condyle, borne 
upon basi-occipital, and showing zc, 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; fr, 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.) 
The high orbito-nasal septuin is a continuous vertical plate of cartilage, upgrowing 
from the tract of the conjoined trabecule. Lateral developments of this ethmoidal wall, in 
