160 GENERAL ORNITHOLOGY. 
and abducent) which move the muscles of tgereyeball ; these holes being collectively about 
equivalent to the foramen lacerum antertus of human anatomy. Parts about the optic foramen, 
before and above, are presphenoidal (figs. 70, 71, ps) and orbito-sphenoidal ; but they are 
obseure to all but the embryologist, and practically furnish no zoélogical characters. 
The Ethmoid (Gr. 76p0s, ethmos, a sieve; fromm the way it is perforated in the human 
species; fig. 62) is the bone of the mid-line of the skull, in front of the sphenoida! elements and 
below the frontal; it is in special relation with the olfactory nervous apparatus, or sense of 
smell. This is not an easy bone to ‘‘ get the hang of” in birds. Referring to figs. 66, 68, eth, 
the student will see in the early embryo a high thin plate of cartilage, the mesethmoid cartilage, 
which is developing lateral processes to form the convoluted walls of the nasal passages. By 
the uprising and forth-growing of the prenasal cartilage, the mesethmoidal plate is tilted back- 
ward, as it were, under the frontal. Next, by absorption of tissue just opposite the future 
eranio-facial suture, the plate is nicked apart, the portion im front of the nick elaborating 
the nasal chambers, which usually remain cartilaginous, and the portion behind this nick 
becoming the permanent plate, fig. 70, eth, pe, to which the name mesethmotd or mid-ethimoid 
is more strictly applicable. Practically, a bird’s ethmoid is chiefly the inter-orbital septum, in 
vertical mid-line between the orbits, with such flange-like processes or lateral plates as may be 
developed to form an orbito-nasal septum separating the eye-socket from the nose-chamber. 
In general, the permanent ethmoidal plate becomes nearly coincident with this orbital wall, and 
pretty well cut off from the osseous or cartilaginous developments, when any, in the nasal cavi- 
ties. It is then fairly under cover of the frontal, with which, as with the sphenoidal elements 
posteriorly, it becomes completely fused. When this inter-orbital septum is fully developed, it 
completely divides the right and left orbital cavities, and its lower horizontal border, fused 
with the basisphenoidal rostrum, may like the latter be thickened by bearing its share of the 
parasphenoidal splint. Oftener, however, this lower border slopes upward and forward, from the 
sphenoidal base to the roof of the skull about the site of the ecranio-facial suture ; and usually 
the septum is incomplete, having a membranous fenestra somewhere near its middle (fig. 70, 
tof). Along the upper border of the mesethmoid plate, or just in the crease between it and 
the overarching frontal may usually be seen a long groove, which, beginning behind at the 
olfactory foramen of the brain-box, conducts the thence-issuing olfactory nerve to the nasal 
chambers. Sometimes there is another such groove, from a similar foramen near by in the 
sphenoidal parts, which similarly traces the course of the ophthalmic (first) division of the tri- 
facial nerve. Occasionally, as in the fowls, the two halves of the frontal bone separate a little 
at the extreme forehead, allowing the mesethmoid plate there to come up flush with the outer 
surface of the skull. 
In some birds, as the low ostrich, for example, the original mesethmoidal cartilage-plate 
does not nick apart into orbital and nasal moieties, but ossifies as a continuous sheet of bone, 
dividing right and left halves of the skull far towards the point of the beak (see fig. 75, beyond 
Rto Pmx). A nasal septum, separated from the orbital septum, may persist to ossify ; form- 
ing, as in the raven, a vertical plate separate from all surroundings, and liable to be mistaken 
for a free vomer (see fig. 79, where the reference line v goes to it, instead of to the truncate 
vomer) ; or, as in many birds, a plate variously auchylosed with its surroundings. But these 
formations, as well as the various turbinal (Lat. twrbo, a whorl) scrolls and whorls formed in 
this part of the skull, belong rather to the organ of smell than to the skull proper. 
The Cranial Bones proper are all those thus far described, excepting the nasal ossifica- 
tions just noted, which belong to the first pre-oral arch; and the stapedial parts of the ear, 
which belong to the hyoidean apparatus (second post-oral arch). Intermediate in some 
respects between the proper cranial bones and 
