234 



COMPARATIVE ANATOMY 



of the middle ear they extend in the order given from the ear drum or 

 tympanum to the oval window or fenestra vestibuli of the internal ear. 

 Thus they serve to carry vibrations from the ear drum to the liquids of 

 the internal ear. Their arrangement is such as to reduce the amplitude of 

 vibrations and to increase their intensity. See Fig. 496. 



quadrate: 

 palato-pterygoid ' 



^ HYOMANDIBULAR 



ARTICULAR- ~ 



DENTARY 



MANDIBULAR 

 A. ELASMOBRANCH 



HYOMANDIBULAR (COLUMELLA > 



QUADRATE 



ARTICULAR_._ 

 DENTARY^ 

 MANDIBULAR. 



2ND 

 MANDIBULAR 



B TELEOST 



STYLOID PROCESS 



(HYOID) 1 



STAPES CHY0MAN0IBULAR3 



INCUS CQUADRATEJ 

 MALLEUS CARTICULAR) 



MECKEL'S 



CARTILAGE CMANDIBULAR) 



HYOID 



DENTARY- 

 STYLOHYOD LIGAMENT CHY0ID3 ■ 



C AMPHIBIAN AND REPTILE. D. MAMMAJL. 



Fig. 190. — Diagrams of the first and second visceral arches in .4, Elasmobranch, B, 

 Teleost, C, Amphibian and Reptile, and D, Mammal, illustrating the transformation 

 of the hinge of the jaw of lower vertebrates into the malleus and incus of the mammal. 

 The third earbone, the stapes, comes from the hyomandibular. (Redrawn after 

 Gegenbaur and Stempell.) 



Development of the Visceral Skeleton. In the human embryo a series 

 of visceral arches separated by pharyngeal pouches appear in relations 

 corresponding to those of aquatic vertebrates. In the first of those arches 

 the maxilla of the upper jaw and the mandible of the lower jaw develop 

 as membrane bones. The mandible, however, surrounds a cartilage, 

 Meckel's cartilage, which corresponds to the mandibula or cartilage of the 

 lower jaw of elasmobranchs. While most of Meckel's cartilage disappears 

 during ontogenesis, that portion which extends into the cavity of the 



