Mandibular and Hyoid arches. The adaptations of both the mandibular and hyoid bars, to functions entirely distinct from those which they primitively served, are most remarkable; and the adaptations of the two bars are in many cases so intimately bound together, that it is not possible to treat them separately.
Fig. 331. View from below of the branchial skeleton of the skull of a Fowl on the fourth day of incubation. (After Parker.)
cv 1. cerebral vesicles; e. eye; fn. frontonasal process; n. nasal pit; tr. trabeculæ; pts. pituitary space; mr. superior maxillary process; pg. pterygoid; pa. palatine; q. quadrate; mk. Meckel’s cartilage; ch. ceratohyal; bh. basihyal; cbr. ceratobranchial; ebr. proximal portion of the cartilage in the third visceral (first branchial) arch; bbr. basibranchial; 1. first visceral cleft; 2. second visceral cleft; 3. third visceral arch.
The most important change of function is undoubtedly that of the mandibular arch, which becomes entirely converted into a skeleton for the jaws. It may be noted as a peculiarity of the mandibular arch that it is never provided with an unpaired basal element.
The simplest forms of metamorphosis are those undergone by Elasmobranchii, of which the Dogfish (Scyllium) and Skate (Raja) have been studied (Parker, No. [456]). In some of these forms, e.g. the Skate, part of the mandibular bar is still related to the hyomandibular cleft (the spiracle).
Elasmobranchii. In Scyllium the hyoid and mandibular arches are at first very similar to those which follow. Soon however each of them sends an anteriorly directed dorsal process ([fig. 329]). The regions which may be distinguished owing to the growth of these processes have received names from ossifications in them which are found in other types. The anterior process of the mandibular arch is known as the pterygo-quadrate bar (Pl.Pt); the dorsal end of the primitive bar from which it starts (M.Pt) is known as the metapterygoid process; while the ventral end of the bar forms the Meckelian cartilage. The upper end of the hyoid arch is known as the hyomandibular.
In a somewhat later stage changes take place which cause these parts practically to assume the adult form ([fig. 327]). The mandibular arch becomes segmented at its bend into (1) a pterygo-quadrate bar (Pl.Pt) which grows forwards in front of the mouth and forms an upper jaw, and (2) a Meckelian cartilage (Mck) which is placed behind the mouth, and forms a lower jaw. The two jaws are articulated together, and the cartilages of the two sides composing them meet each other distally.
At the articulation of the Meckelian cartilage with the quadrate part of the pterygo-quadrate is situated a ligament (M.Pt), which takes the place of the metapterygoid process of the previous stage, and passes up on the anterior side of the spiracle, to be attached to the cranium in the front part of the auditory region. This ligament, which is supplemented by a second ligament, the ethmo-palatine ligament, passing from the pterygo-quadrate bar to the antorbital region of the skull, is not the most important support of the jaw. The main support is, on the contrary, given by the hyoid arch; the hyomandibular segment of which (H.M) as well as the adjoining segment (ceratohyoid C.Hy) are firmly attached by ligament to the mandibular arch. The hyomandibular is articulated with the cranium beneath the pterotic ridge (Pt.O).
In the type just described, the hyoid and mandibular arches undergo less modification than in almost any other case. The hyoid arch has altered its form, but retains its respiratory function. It has however acquired the secondary function of supporting the mandibular arch. The mandibular arch is divided into two elements, which form respectively the upper and lower jaws. It is not directly articulated with the skull, and its mode of support by the hyoid arch has been called by Huxley (No. [445]) hyostylic.