128 Basic Structure of Vertebrates 



ration of the middle region of the outer series of bones from the inner 

 series (Fig. 121B). The detached region becomes more or less convex 

 outward, forming the zygomatic arch, which springs outward from 

 the maxilla and posteriorly joins the quadrate or (in mammals) the 

 squamosal. The greater part of the arch consists of the jugal (malar) 

 bone. The space between the zygoma and the lateral wall of the cra- 

 nium is occupied by the bulky elevator muscles of the lower jaw, 

 which extend from the coronoid region of the jaw to the posterolateral 

 surface of the cranium. 



In mammals the quadrate and articular do not appear as 

 bones of the adult skull. The all-dermal lower jaw articulates with 

 the dermal squamosal above. Embryonic vestiges of the quadrate 

 and articular become converted into parts of the accessory auditory 

 mechanism of the ear (Fig. 520). This is a radical departure from 

 the arrangement which exists in all amphibians, reptiles, and birds. 



Reviewing the whole vertebrate series, three markedly different 

 arrangements of the jaws may be recognized: (1) upper jaw and hyo- 

 mandibular suspensor swung in V shape below cranium, as in many 

 fishes; (2) no suspensor; upper jaw incorporated into roof of mouth 

 and (with certain exceptions) immovably attached to cranial bones; 

 joint between articular and quadrate; this arrangement characteristic 

 of amphibians, reptiles, and birds; (3) as in (2), but joint between 

 dentary and squamosal; the arrangement peculiar to mammals. 



In lizards, snakes, and birds, more or less mobility in the upper 

 jaw is secured by various specialized arrangements of the jawbones. 



Appendicular Skeleton 



paired appendages 



The paired locomotor appendages are the pectoral and pelvic fins 

 of fishes and the pectoral and pelvic limbs of the four-footed verte- 

 brates or "tetrapods." Whether fin, leg, wing, or arm, the skeleton 

 of the appendage consists of a girdle, lying in the body-wall and 

 capable of relatively little or no movement, and serving as a foundation 

 for attachment of the skeleton of the externally projecting structure 

 which is, in a more literal sense, an "appendage" of the body (Fig. 

 102). The appendage is more or less elaborately jointed and freely 

 mobile. The pectoral and pelvic appendages, including girdles, are 

 constructed on the same general plan, differing merely as to details. 



A simple type of fin appears in sharks (Fig. 122). The pectoral 

 girdle is a stout transverse cartilaginous bar, convex downward, 

 situated in the ventral body-wall just behind the branchial region. 

 Laterally, it is prolonged upward as a tapering process. By analogy 



