ENDOSKELETON AND VOLUNTARY MUSCLE 59 



At the tip of each half of the lower jaw is a short segment, the 

 mentomeckelian bone, directly beneath the corresponding pre- 

 maxillary bone in the upper jaw. The mentomeckelian bone 

 continues backward to the angle of the jaw as Meckel's cartilage, 

 passing between two overlapping bones, the dentate in front and 

 outside of the cartilage, and the angulare, inside of and below the 

 cartilage. The posterior half of the cartilage runs in a groove 

 on the upper surface of the angulare, widening out to form an 

 articulating surface at the end of the jaw. In prepared skeletons, 

 the cartilage is often absent, but the groove in the angulare can be 

 clearly seen. There are no teeth in 

 the lower jaw. The coronoid process 

 of the angulare offers a point of attach- 

 ment for muscles that close the jaw. 



The tips of both upper and lower | §|f J .alp 



jaws are movable. When the tip of 

 the lower jaw is thrust upward, the 



premaxillaries are raised, pushing the ss ^ £/ \j| w PI R 



pref acial processes of the latter against 

 the sides of the external nares, closing 



them. Fig. 34.— Hyoid apparatus of 



HyOld Skeleton. — The frog in the Rana catesbeiana, ventral view. 



,11 , • • i i -ji •ii a.c, anterior cornu; a. p., an- 



tadpole stage is provided with gills terior process . al p f alar p y pro . 

 and gill clefts, perforating the pharynx cess; p.i.p. posterior lateral 

 on either side. When metamorphosis process; **" thyroid procesa - 

 occurs, the gills disappear and the branchial skeleton supporting 

 the gills becomes converted into the hyoid skeleton of the adult, 

 most of which is cartilaginous (Fig. 34). The body of the hyoid 

 is a flat plate of cartilage, roughly quadrilateral in shape, located 

 in the floor of the mouth cavity. The anterior cornua of the hyoid 

 are a pair of slender rods curving backward and upward, one to 

 each prootic bone. The alary processes extend laterally, one on 

 each side just behind the corresponding anterior cornu. The 

 posterior lateral processes extend from the body behind and at the 

 sides. Between them are the ossified thyroid processes which 

 enclose and support the larynx. 



Chondrocranium. — The primitive vertebrate skull was cartila- 

 ginous. In higher vertebrates parts of this chondrocranium are 

 ossified or replaced by membrane bone. In the frog much of the 

 cartilaginous cranium remains. The exoccipitals, prootics, and 



