THE SKULL. 575 



degenerated, in correlation with the total disappearance of a 

 branchial respiration at all periods of life. Their remnants 

 become more or less important parts of the hyoid bone, and are 

 solely employed in support of the tongue. Their basal portions 

 are best preserved, forming parts of the body of the hyoid. The 

 posterior (thyroid) cornua of the hyoid are remnants of the true 

 arches. Of these there are two in the Chelonia and Lacertilia, 

 and one in the Aves and Mammalia. In Aves the cornu formed 

 from the first branchial arch (fig. 331, cbr) is always larger than 

 that of the true hyoid arch (ch}. 



Mandibular and Hyoid arches. The adaptations of both 

 the mandibular and hyoid bars, to functions entirely distinct from 



FlG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL 



OF A FOWL ON THE FOURTH DAY OF INCUBATION. (After Parker.) 

 cv i. cerebral vesicles ; e. eye ; fu. frontonasal process; n. nasal pit; tr. trabeculze; 

 pts. pituitary space ; mr. superior maxillary process ; pg. pterygoid ; pa. palatine ; 

 q. quadrate ; mk. Meckel's cartilage ; ch. cerato-hyal ; bh. basihyal ; cbr. cerato- 

 branchial ; ebr. proximal portion of the cartilage in the third visceral (first branchial) 

 arch; bbr. basibranchial ; i. first visceral cleft; 2. second visceral cleft; 3. third 

 visceral arch. 



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. 



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 



