160 GENEBAL OBNITHOLOGY. 



and abducent) which move the muscles of the eyeball; these holes being collectively about 

 equivalent to the foramen lacerum anterius of human anatomy. Parts about the optic foramen, 

 before and above, are presphenoidal (figs. 70, 71, ps) and orbito-sphenoidal ; but they are 

 obscure to all but the embryologist, and practically furnish no zoological chajacters. 



The Ethmoid (Gr. rjdfios, ethmos, a sieve ; from the way it is perforated in the human 

 species ; fig. 62) is the bone of the mid-line of the skuU, in front of the sphenoidal 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. Keferring to figs. 66, 68, eth, 

 the student wUl see in the early embryo a high thin plate of cartilage, the mesethmoid cartilage, 

 which is developing lateral processes to fonn 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 

 cranio-facial suture, the plate is nicked apart, the portion in 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 mesethmoid or mid-ethmoid 

 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 skuU about the site of the cranio-facial suture ; and usually 

 the septum is incomplete, having a membranous fenestra somewhere near its middle (fig. 70, 

 iof). 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 

 B to 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 anchylosed with its surroundings. But these 

 formations, as well as the various ty/rlinal (Lat. twrbo, a whorl) scrolls and whorls formed in 

 this part of the skull, belong rather to the organ of smeU than to the skull proper. 



The Cranial Bones proper are aU 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 



