108 The Vertebrate Organ Systems 



As each rib slopes downward and outward, the chest cavity is 

 larger at the posterior end than at the anterior. The ventral articula- 

 tion is always much lower than is the dorsal one. 



The breastbone or sternum consists primarily of a series of small 

 bones, the sternebrae. In man, many of these sternebrae have become 

 fused to form the gladiolus. The anterior portion remains somewhat 

 separated as the manubrium, while posteriorly there is a small carti- 

 laginous process, the xiphoid process. 



THE VISCERAL SKELETON 



Many seemingly unrelated structures such as portions of the 

 upper and lower jaws, the ear ossicles, the cartilages of the larynx, and 

 the bone supporting the tongue make up the visceral skeleton of most 

 verebrates. These are grouped together because they originate from 

 similar structures, that is, gill arches. In fish, the first arch has been 

 transformed into the jaw, but the others are still functional as sup- 

 porting structures for the gills. Gills are not present in the adult 

 terrestrial vertebrates, but the gill arches still form in the embryo 

 and assume other functions in the adult. The term visceral skeleton 

 is appHed to these structures because of their original association with 

 the respiratory tract which is part of the internal organs, the viscera. 



The first gill arch forms the cartilaginous elements of the upper 

 and lower jaws. Among most vertebrates, these cartilaginous ele- 

 ments are covered by the investing dermal bones, and are completely 

 concealed. This situation is particularly true in mammals where the 

 cartilage of the gill arch completely disappears in the adult jaw. Part 

 of the first gill arch also forms two of the ear ossicles of the middle 

 ear. The third ear ossicle is derived from a portion of the second 

 gill arch. In addition, this latter arch together with the third forms 

 the hyoid apparatus, which supports the tongue. The fourth and fifth 

 arches give rise to the cartilages of the larynx. Other gill arches 

 give rise to the cartilages of the epiglottis and trachea. 



THE APPENDICULAR SKELETON 



The bones of the appendages and the girdles to which they are 

 attached form the elements of the appendicular skeleton (Fig. 30). 

 Each girdle is formed from three pairs of bones, while the appendages 

 have a series of bones which are variously modified. Among those 

 vertebrates with appendages, there are many modifications which are 



