414 



TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



homologous with the Meckelian bar of the mandible of the shark and represented by the 

 endosteal portion of the articular bone. Each half of the mandible forms a simple lever of 

 the third class, pivoted on the quadrate bone by a hinge-like joint (Fig. 284). The main 



Fulcrum 



Fig. 284. The Mandible as a Lever of the Third Class. Skull of Sphyrana after removal of part of the premaxilla and maxilla 



tendons of the jaw muscles are inserted in front of the pivot and behind the teeth, through 

 which the power is applied to the resistence (Fig. 285). The jaw muscles are briefly noted 

 below (p. 425). 



The inner upper jaw and the mandible are attached to the cranium (Fig. 284) first, by a 

 joint between the ethmo-vomer block and the palatine bone, secondly, through the hyo- 

 mandibular, the metapterygoid and the symplectic bones, which support the quadrate. 

 The head of the hyomandibular (Fig. 119) is movably attached to the upper outer rim of the 

 otic capsule by two sockets, the anterior one borne jointly by the sphenotic and the prootic, 

 the posterior by the pterotic. The lower segment of the hyomandibular usually tapers 

 down to form the symplectic, which arises from a separate center of ossification; the sym- 

 plectic is wedged into a groove on the posterior end of the quadrate. It appears to be only 

 a process of the hyomandibular. Apparently the wedging of the symplectic into the quad- 

 rate stiffens the latter against the thrusts and pulls of the mandible, while the unossified 

 joint between the hyomandibular and the symplectic permits both to grow in length. 

 Just above the symplectic, on the medial surface, there is a movable ball-and-socket joint 

 with the interhyal or stylohyal, which suspends the lower parts of the hyoid arch from 

 the hyomandibular. 



Hence the hyomandibular suspends the upper and lower jaws in front and the lower 

 segments of its own arch below. Collectively, the palato-pterygo-quadrate series and the 

 hyomandibular-symplectic act like a simple V-truss in resisting thrusts of the mandible 

 (Fig. 284). Lateral warping and vertical stretching of the hyomandibular-symplectic- 

 quadrate series is prevented by the lunate preopercular, which is wrapped tightly around 

 the back of the quadrate-symplectic and hyomandibular and is often stiffened with promi- 

 nent ribs. 



The metapterygoid, although often thin, connects the upper part of the palato-pterygo- 

 quadrate arch with the hyomandibular. It helps to strengthen the whole arch and to give 



