THE OSSEOUS MANDIBLE. 



29 



immovably united with the OS articulare. Hence the complete 

 ramus of the mandible articulates directly with the skull in 

 mammals, but only indirectly, or through the intermediation 

 of the quadrate, in other Vertehrata, In birds and reptiles, 



f-mx;' 



Fig. 9. — The head of a fcctal Lamb dissected so as to show Meckers cartilage, 31 ; the 

 malleus, w ; the iiicus, i; the tympanic, Ty ; the hyoid, //; the squamosal, Sq ; 

 pterygoid,/"^; palatine,^Z; laclirj'mal, Z ; premaxilla,^?«a!; nasal sac, it^; Eustachian 

 tube, Eu. 



the proximal end of the quadrate bone articulates directly 

 (with a merely apparent exception in Opliidki)^ and indepen- 

 dentl}'' of the hyoidean apparatus, with the periotic capsule. 

 In most, if not all fishes, the connection of the mandibular 

 arch with the skull is effected indirectly, by its attachment to 

 a single cartilage or bone, the liyomcmdibular^ w^hich repre- 

 sents the proximal end of the hyoidean arch {see Fig. 24). 



The ossification of the hyoidean apparatus varies immense- 

 ly in detail, but usually gives rise to bony lateral arches, and 

 a median portion, bearing much the same relation to them as 

 the sternum has to the ribs. When the lateral arches are com- 

 plete, they are connected directly with the periotic capsule. 



The proximal end of the hyoidean arch is often united, 

 more or less closely, with the outer extremity of the bone, 

 called columella auris^ or stapes^ the inner end of which, in 

 the higner ^^ertehrata^ is attached to the membrane of the 

 fenestra ovalis. 



In ordinary fishes, a fold of the integument extends back- 

 ward from the second visceral arch over the persistent bran* 



