156 HISTORY OF THE HUMAN BODY 



series, will considerably increase the number of original vis- 

 ceral arches and render more probable their descent from a 

 form like Amphioxus. 



In the other Orders of fishes the visceral skeleton becomes 

 somewhat modified, but the seven pairs of visceral arches are 

 recognizable in all cases. The most marked changes are those 

 affecting the jaws, and are primarily due to the extensive de- 

 velopment of dermal bones, which reinforce the cartilaginous 

 bars, as they do in the case of the chondro cranium. ^The 

 lower jaw becomes almost entirely encased by them, the 

 principal dermal elements being the dentary, which covers the 

 outside and bears the most or all of the teeth, and the angu- 

 lare, which covers the inner side. 



Lying within these, as if bound in splints, is the cartilagi- 

 nous lower jaw, the original visceral arch, which is destined 

 from now on to lose its functional importance save at its 

 posterior end, which here emerges 'from the splints and pre- 

 sents a rounded articular surface. This piece is called the 

 articulare, and sometimes ossifies, forming a cartilage bone. 

 The entire cartilaginous arch, thus subordinated, the mandibu- 

 lar cartilage, is also called Meckel's cartilage, the name com- 

 memorating the distinguished German anatomist, Johann 

 Friedrich Meckel [1781-1833], who first saw it in the embryo 

 human jaw, lying encased in the dermal bones, much as in the 

 adult ganoid or teleost. 



The original upper jaw, however, loses still more prestige, 

 since its function as a jaw is entirely usurped by a set of 

 dermal bones, the pr&maxillary and maxillary, placed paral- 

 lel to, but outside of it. In spite of this, however, it becomes 

 directly encased by other dermal bones, the palatine and the 

 pterygoid, and is retained as an accessory upper jaw in some 

 fishes and a few salamanders. Its posterior end, like that of 

 the lower jaw, remains free and, after the reduction of the 

 anterior portion, fits in between the hyomandibular and the 

 articulare of the lower jaw as an extra suspensory piece. In 

 later development it ossifies as a cartilage bone, under the 

 name of quadrate. As for the ultimate fate of the anterior 



