Page 358 - My FlipBook

368 DENTAL ANATOMY.
terior portion of this arch are several broad, flat, scale-like bones which
cover the gills and are called opercular bones. The upper posterior one
is the operculum. The one in front of this, presenting a curved outline
anteriorly and a ])osterior serrate border, is the preoperculum, while the
two beneath are the interoperculum and suboperculum respectively. The
arch itself is composed, first, of the hyo-mcmdibular bone (Fig. 190,
hy), which by its proximal extremity is attached to the side-wall of the
cranium, being lodged in a distinct oblong socket ; secondly, of the
(piadrate (qu, Fig. 190), Avhich articulates with it by suture at its lower
extremity ; thirdly, the symplectic, a small splint occupying a groove
in the inner side of the quadrate ; and, lastly, the lower jaw, which is
movably articulated with the quadrate and which normally supports
teeth. Each half is made up of the dcntary or tooth-bearing piece,
meeting its fellow of the opposite side in the median line or symphysis,
and an articular piece which connects the dentary with the quadrate.
To this may be added the coronoid, a small bone superimposed above
the junction of the articular and dentary, and an cnif/ular which lies
just beneath the articulation of the quadrate and articular.
From the region of the quadrate another chain of bones extends
upward, forward, and inward to the anterior part of the roof of the
mouth, where it is attached by ligament to the side of the vomer, or
that bone a\ hich forms the prominent rostrum of the cranium after the
removal of the arches. This chain is known as the pokdo-qvadj-ate
arch, and the bones entering into its composition are the eido-, mefto,
ecto-pteryr/oid.s and the palatine. The ento-pterygoid is applied to the
hyo-mandibular and quadrate upon their anterior margins ; the meso-
pterygoid starts out from the quadrate and ento-pterygoid, and extends
toward the vomer, where it meets the palatine, which completes the
arch. The ecto- pterygoid lies above the junction of the meso-pterygoid
and the palatine (Fig. 190).
Immediately in front of the vomer, and attached to it and to the pala-
tines, are two considerable bones which project downward and backward,
bounding the u}>]ier posterior portion of the canthus of the mouth—the
superior maxi/knies. In front of these, again, are the pre- ovmfermax-
illaries, limiting the anterior boundary of the oral cavity above.
Another bone, which in some forms (ex. catfishcs) reaches the roof of
the mouth, needs to be noticed in this connection. The suborbital
ring, or those bones which encircle the orbit below, articulates by its
most anterior piece (lachrymal) with a bone suturally united to the
cranium and taking part in the boundary of the orbit in front and
above. This is the prefrontal, and, as already remarked in the cat-
fishes, owing to the width of the mouth takes part in the formation of
its bony roof, and in some species bears teeth. This bone is frequently
mistaken for the vomer, but, as I have recently ascertained, is certainly
tlie ]n-efrontal, Avhich must likewise be added to the category of tooth-
sup]iorting bones in fishes.
The several arches and bones so far enumerated, with the exception
of the sca])ular arcli—wliich never, to my knowledge, is dentigerous—are
in direct relation with the mouth, and are exclusively concerned in pre-
hensile and crushing functions ; but those which are to follow, especially