THE ENDOSKELETON 715 



A small scale-like bone, the jugal, is attached to the posterior end of 

 the maxilla. 



The lower jaw is quite similarly modified. The articulare articu- 

 lates with the quadrate and is continued forward by a narrow pointed 

 rod of cartilage which is really the unossified distal end of the primary 

 Meckel's cartilage (the primary lower jaw). The articulare is the ossi- 

 fied proximal end, therefore, a replacing bone. Then there is a large 

 toothed investing bone which ensheaths Meckel's cartilage and forms 

 the main part of the secondary lower jaw. This is the dentary. There 

 is also a small investing bone, the angular, which is attached to the 

 lower and hinder end of the articulare. 



The upper jaw connects with the cranium partly by the articulation 

 of the palatine with the olfactory region and partly by means of a sus- 

 pensorium formed of two bones separated by a cartilaginous interval. 

 The larger, usually called the hyomandibular, articulates with the audi- 

 tory capsule by a facet and the small, pointed symplectic fits into a 

 groove in the quadrate. Both bones are attached by fibrous tissue to 

 the quadrate and metapterygoid, and in this way the suspensorium and 

 palatoquadrate together form an inverted arch which articulates freely 

 in front with the olfactory, and behind with the auditory capsule. This 

 gives rise to an extremely mobile upper jaw. 



The chondrocranium is a solid one-piece capsule which completely 

 encloses the brain and the principal sense organs. The cranium proper 

 is fused with paired nasal capsules and paired auditory capsules. 



Closely associated with the skull, but not fused with it, is the man- 

 dibular skeleton, consisting of an upper jaw (pterygoquadrate cartilages) 

 and a lower jaw (Meckel's Cartilage). Back of the jaw are the visceral 

 arches. These are composed of upper and lower parts like the jaws. 

 The first pair is specialized as the hyoid arch, the five others being the 

 more generalized branchial arches that afford support for the gills. 



AMPHIBIA 



The skull (Fig. 410) Articulates with the atlas by two condyles 

 which are formed by the exoccipitals. There is an auditory columellar 

 apparatus fitting into the fenestra ovalis. 



AVES 



The skull (Fig. 412) is rounded, has large orbits, and the facial 

 bones are extended out upon the beak. The quadrate is movable and 

 articulates with the squamosal. There is a single occipital condyle. 

 There are no teeth in modern forms. The cervical vertebrae have pad- 

 dle-shaped articular surfaces which give the neck great flexibility, and 

 thus make the beak a very versatile instrument. 



