The external auditory meatus, tympanum and Eustachian tube are remains of the hyomandibular cleft, the membrana tympani being a remnant of the cleft membrane and therefore lined by ectoderm outside and entoderm inside.
Gadow's view, which is one of the latest, is that all three are derived from the hyomandibularplate which connects the dorsal ends of the hyoid and mandibular bars (Anatomischer Anzeiger, Bd.
The pinna is formed from six tubercles which appear round the dorsal end of the hyomandibular cleft or, more strictly speaking, pouch.
The first or hyomandibular pouch, placed between the mandibular and hyoid arches, has rather the character of a double layer of hypoblast than of a true pouch, though in parts a slight space is developed between its two walls.
A well-marked groove is present between the mandibular and opercular arches, but so far as we can make out it is not a remnant of the hyomandibular cleft.
The section in front of the hyomandibularcleft stands in a peculiar relation to the two branches of the fifth nerve.
It forms a flattened cavity, dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft, Pl.
My material of the stages when it should be present is somewhat scanty, but I have as yet, very likely owing to the imperfection of my material, been unable to find Scott's hyomandibular pouch either in my sections or surface-views.
The development of the true auditory vesicle will be considered separately from that of the accessory structures derived from the hyomandibular cleft.
The first of these will form the hyomandibular cleft, the second the hyobranchial, and the third, fourth and fifth the three branchial clefts.
In the youngest larva figured the ventral part of the hyomandibular cleft is already closed: the dorsal part of the cleft is destined to form the spiracle (sp).
It is still uncertain whether there is any distinct element corresponding to the hyomandibular of fishes.
Pt); while the whole arch becomes firmly bound to the hyomandibular (H.
Skate, part of the mandibular bar is still related to the hyomandibular cleft (the spiracle).
The main support is, on the contrary, given by the hyoid arch; the hyomandibular segment of which (H.
The letter tv pointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated.
There can be but little doubt that the hyomandibular diverticulum gives rise, as in the Amniota, to the Eustachian tube and tympanic cavity, except when these are absent (i.
Its upper element ossifies as the hyomandibular (fig.
The malleus and incus are at first embedded in the connective tissue adjoining the tympanic cavity (hyomandibular cleft, vide p.
This groove is connected with the hyomandibular cleft, but I have not determined whether it is now perforated.
The gill slits also become perforated, but the hyomandibular diverticulum in most species never actually opens to the exterior, and in all cases becomes very soon closed.
The pterotic and sphenotic together give rise to a large concave surface by which the hyomandibular articulates with the cranium.
In this case the mandibular arch becomes connected with the hyomandibular and supported by the hyoid arch.
The structure is as a whole homologous with the auditory ossicles of mammals and the hyomandibular of fish.
The latter is much reduced, and is converted into the tympanic ring, while the hyomandibular of fish is represented by the auditory ossicles[108].
The posterior surfaces of both hyomandibular and cerato-hyal bear slender cartilaginous processes, the =gill rays=.
In some Rays the hyoid is attached to the posterior face of the hyomandibular near its proximal end, and may even come to articulate with the cranium.
The hyoid and branchial arches are much reduced; and the representative of the hyomandibular is connected with the auditory apparatus, forming the auditory ossicles[66].
The =hyoid arch=, which consists of the hyomandibular and the hyoid, and bears branchiostegal rays on its posterior face.
Behind the hyomandibular there is a large bony plate, the =operculum=, formed of four large membrane bones.
The unossified tract between the hyomandibular and symplectic is continuous in front with a strong bar, which remains partly cartilaginous and is partly converted into cartilage bone.
The columella and stapes are together homologous with the mammalian auditory ossicles and with the hyomandibular of Elasmobranchs.
It is connected to the hyomandibular by a thick symplectic ligament containing a small bone homologous with the symplectic of Teleosteans.
The palato-pterygo-quadrate bar and hyomandibular are fused to the cranium, and Meckel's cartilage articulates directly with the part corresponding to the quadrate.
Defn: Pertaining both to the hyoidean arch and the mandible or lower jaw; as, the hyomandibular bone or cartilage, a segment of the hyoid arch which connects the lower jaw with the skull in fishes.
In the shark it is said to be hyostylic, the hyomandibular intervening.
The slender bone which thus keys together the upper and lower elements, hyomandibular and quadrate, forming the suspensorium of the lower jaw, is known as symplectic (18).
The hyomandibularis thought to be homologous with the stapes, or stirrup-bone, of the ear in higher animals.
In Dicranodus texensis the palato-quadrate articulates with the postorbital process of the cranium, as in the Hexanchidae, and the hyomandibular is slender.
In the Chimaera there is no suspensorium, the mandible being directly attached to the cranium, of which the hyomandibular and quadrate elements form an integral part, this arrangement being called autostylic.
In the sharks and in all higher fishes the mandible is joined to the skull by a suspensorium of bones or cartilages (quadrate, symplectic, and hyomandibular bones in the Teleost fishes).
In the chimaera the mandible is articulated directly with the skull, the hyomandibularand quadrate elements being fused with the cranium.
In addition to the bones already enumerated there is present a ring of circumorbital bones, a preopercular, behind and external to the hyomandibular and quadrate, and squamosal, external to the hinder end of the auditory capsule.
From the particular skeletal structures which develop in the visceral arches bounding it the anterior cleft is known as the hyomandibular cleft, the next one as hyobranchial.
In common usage the hyomandibular cleft is called the spiracle, and the series of clefts behind it the branchial clefts.
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