The lower jaw or mandible consists of a pair of cartilaginous rods (Meckel's cartilages) in connection with each of which there are developed two membrane bones and one cartilage bone. The cartilage bone is the mento-meckelian (fig. 26, A, 14), a very small ossification at the extreme anterior end. The membrane bones are the angulo-splenial and the dentary. The angulo-splenial is a strong flat bone which forms the inner and lower part of the mandible for the greater part of its length. Its dorsal surface is produced into a slight coronoid process. The dentary (fig. 26, A, 15) is a flat plate which covers the outer surface of the anterior half of the mandible, as far forwards as the mento-meckelian. The lower jaw is devoid of teeth. The part of Meckel's cartilage which in most vertebrates ossifies, forming the articular bone, remains unossified in the Frog.
The Hyoid apparatus.
The hyoid of the adult Frog is formed of the modified hyoid and branchial arches of the tadpole. It consists of a broad thin plate of cartilage, the basilingual plate (fig. 29, B, 1), drawn out into two pairs of long processes, the cornua. The basilingual plate is broader in front than behind, and is formed from the fused ventral ends of the hyoid and branchial arches of the tadpole.
The anterior cornua (fig. 29, B, 2) form a pair of long slender cartilaginous rods which project from the body of the hyoid at first forwards, then backwards, and finally upwards and somewhat forwards again, to be united to the auditory capsules just below the fenestrae ovales. They are formed from the dorsal portion of the hyoid arch of the tadpole and are homologous with the cerato-hyals of the Dogfish.
The posterior cornua form a pair of straight bony rods diverging outwards from the posterior end of the basilingual plate. They are formed from the fourth branchial arches of the tadpole, and differ from the rest of the hyoid apparatus in being well ossified.
The columellar chain, which has been already described (p. 157), should be mentioned with the hyoid as it is homologous to the hyomandibular of fishes.
The sternum of the Frog, though regarded as part of the axial skeleton, is so intimately connected with the pectoral girdle, that it will be described with the appendicular skeleton.
2. The Appendicular Skeleton.
This consists of the skeleton of the two pairs of limbs and their respective girdles. It is at first entirely cartilaginous but the cartilage becomes later on mainly replaced by bone. The only bone developed in connection with the appendicular skeleton, which has no cartilaginous predecessor, is the clavicle.
A. The Pectoral girdle.