28 COMPARATIVE ANATOMY 



preformed in cartilage. But, as a matter of fact, in the frog it 

 is a membrane bone, which largely or wholly replaces the 

 ossification in cartilage. The same is the case with a large 

 three-headed bone, the pterygoid, which reaches from the point 

 where the palatine unites with the maxilla to the auditory 

 process of the skull, and gives off outward and backward 

 a process towards the articular surface for the lower jaw. 

 This posterior limit of the pterygoid lies beneath a rod of 

 cartilage which extends from the pro-otic region outward and 

 backward, and is known as the quadrate cartilage. Exter- 

 nally it presents a concave articular surface, the glenoid cavity 

 for the lower jaw ; it is covered below by the process of the 

 pterygoid as described, and above by a T-shaped bone, the 

 squamosal. Finally, the posterior end of the maxilla is 

 connected with the outer end of the quadrate cartilage by a 

 small splint-shaped bone, the quadrato-jugal. It should be 

 noticed that the palatine and pterygoid, with the cartilage 

 which overlies them, form the front and exterior borders of a 

 cavity which is completed internally and posteriorly by the 

 cranium and auditory process. In this cavity the eye is 

 lodged, and it is known as the orbital cavity, or, more shortly, 

 the orbit. It is open below, so that the eyeballs are only 

 separated from the mouth-cavity by the integuments lining the 

 latter and by a thin sheet of muscle. The lower jaw or 

 mandible consists of two bars of cartilage, one on each side, 

 frequently known as Meckel's cartilages, united in the middle 

 line in front by ligament. Each cartilage ends in front in a 

 little ossification, the mento-Meckelian (cartilage-bone), and is 

 ensheathed by two membrane bones, the angulo-splenial, 

 inside and below, and the dentary covering the upper and 

 outer sides of the anterior half of Meckel's cartilage. The last 

 named widens out behind and forms the articular head which 

 fits into the glenoid cavity of the suspensorium. 



The Branchial skeleton is very much reduced in the adult 

 frog, and comprises what is known as the hyoid apparatus. 

 This lies on the ventral surface of the throat and consists of a 

 broad thin cartilaginous plate, the body of the hyoid, from 

 which processes are given off anteriorly and posteriorly. The 

 anterior processes or cornua are cartilaginous and run first 

 forward and then upward and backward as two slender 

 curved cartilaginous rods which are united with the cartilage 



