SKELETON. 



73 



In the hyoid and branchial arches ossification occurs to a greater or 

 less extent, the resulting cartilage bones having the same names as the 

 corresponding cartilages. There are never any membrane bones in 

 this region. In the teleosts the hyomandibular ossifies as two bones, a 

 dorsal hyomandibular and a lower symplectic which connects with the 

 quadrate. There is, however, a considerable amount of union between 

 the various arches in the adults of all tetrapoda, where the branchial 

 respiration is lost and the arches have to assume other functions than 

 the support of gills. 



The mode of suspension of the 

 jaws varies. In a few elasmobranchs 

 the pterygoquadrate articulates di- 

 rectly with the cranium (amphistylic) ; 

 in others it is suspended by ligament 

 and by the interposition of the 

 hyomandibular between the otic cap- 

 sule and the hinder end of the jaw 

 (hyostylic) ; while in all groups above 

 the fishes the pterygoquadrate is more 

 or less completely fused with the 

 cranium (autostylic). 



The ear bones or ossicula audi- 

 tus are best treated together here, 

 although their consideration requires 

 the mention of structures not yet de- 

 scribed. The ear bones occur only 

 in the tetrapoda: they present several P ter yg id muscle; o, oral cavity;^, 



j ** . parotic process; pt, pterygoid bone; 



modifications not readily homologized s, stapes; t, tympanic cavity; tm, 



with each other, though they all have ******* membrane - 

 the same function of conveying sound waves across the tympanum 

 to the inner ear. In all there is an opening, the fenestra vestibuli 

 (f. ovale) in the lateral wall of the otic capsule, which is occu- 

 pied by a movable bone, the stapes, of uncertain homologies, but 

 probably representing the hyomandibular of the fishes, which otherwise 

 is lacking in all tetrapoda. This view is the more probable since in 

 some vertebrates the stapes is connected developmentally with the rest 

 of the hyoid arch. 



In urodeles and caecilians a slender process extends from the quad- 

 rate across the poorly developed tympanic cavity to articulate with the 



FIG. 70. Diagram of the middle 

 ear of a lizard, after Versluys. a, 

 articulare;c, columella; ec, extracolu- 

 mella; h, hyoid; ie, inner ear; mpt, 



