STRUCTURE OF THE VERTEBRATES 



217 



The next advance is in the reptiles. Tlie arytenoids are of in- 

 creased size, and the sixth visceral arches form an epiglottis. 

 The paired cartilages migrate toward the ventral side of the 

 glottis and unite to form the epiglottis which curves over the 

 opening of the glottis. In addition, the third visceral (first 

 branchial) cartilages fuse with the hyoid bone, forming the 

 greater horns of the hyoid, and bringing the structure into close 

 contact with the larynx. Another modification is the fusion of 

 the anterior rings of the trachea to form a supporting cricoid 

 cartilage of the larynx. 



Epiglottis, 



Arytenoid 

 Glottis 



Vocal cords 



Trachea 



Hyoid- 



Thyroids 

 cartilage 



Arytenoid- 

 Cricoid 



A. Necturus (Amphibia) B. Tortoise (Reptilia) C. Opossum (Mammalia) 



Fig. 127. Development of the Larynx. 



In the mammals the remaining cartilages of the branchial 

 skeleton are brought into the larynx. The second and third 

 (fourth and fifth visceral) cartilages unite across the ventral 

 side of the larynx to form the thyroid cartilage. In the mono- 

 tremes this cartilage is clearly separated into two parts. A simi- 

 lar division is found in the embryos of the higher mammals, but 

 in these groups a complete fusion takes place during development. 



The homologies of the first two visceral arches were given in 

 the section on the Visceral Skeleton, but are included here for 

 clarity. The jaws and cartilaginous supports for the gills as- 

 sumed their definitive condition in the elasmobranch fishes. 

 Therefore, the posterior five visceral cartilages are called the 

 branchial cartilages, and Visceral III is the same as Bran- 

 chial I. 



