XIII 



PHYLUM CHORDATA 



139 



composing the visceral arches (Figs. 768 and 769). These are in- 

 complete hoops of cartilage, mostly segmented, which lie in the 

 sides and floor of the mouth-cavity or pharynx. The first of these 

 forms the upper and lower jaws. The upper jaw, or palato-quadrate 

 ( pal. q.}, consists of two stout rods of cartilage firmly bound to- 

 gether in the middle line and bearing the upper (or anterior) series 

 of teeth. The lower jaw, or Meckel's cartilage (Mck.\ likewise con- 

 sists of two stout cartilaginous rods firmly united together in the 

 middle line, the union being termed the symphysis. At their outer 



rn.ck 



hyp.b, 



ce.r.br.3 



cer.br: t 



c&r.br.s 



ph.br. 



Kn;. 709. Chiloscy Ilium, ventral view of the visceral arches. Letters as in preceding figure. 

 In addition, l>. l . basi-branchial plate ; ccr. In: cerate-branchiate ; hyp. In: hypo-branchials. 



ends the upper and lower jaws articulate with one another by a 

 movable joint. In front the upper jaw is connected by a ligament 

 with the base of the skull. 



Immediately behind the lower jaw is the liyoid arch. This con- 

 sists of two cartilages on each side, and a mesial one in the middle 

 below. The uppermost cartilage is the hyo-mandibular (hy. mn.) : 

 this articulates by its proximal end with a distinct articular facet 

 on the auditor}' region of the skull ; distally it is connected 

 by ligamentous fibres with the outer ends of the palato-quadrate 



