XIII 



PHYLUM CHORDATA 



is 



429 



Bird. When the mandible 



depressed, the contraction of the 



digastric muscle causes a for- 

 ward movement of the lower end 



of the quadrate, which pushes 



forwards the rnaxillo-jugal bar 



and the palatines and ptery- 



goids, the latter sliding upon 



the rostrum. Both the maxillae 



and the palatines are articulated 



in front with the premaxilla, and 



together push it upwards; in 



this way depression of the lower, 



produces an automatic raising of 



the upper, jaw. The great size 



and strength of both premaxilla 



and mandible are remarkable, as 



also is the fact that the orbit is 



completely surrounded by bone, a 



backward process of the lacrymal 



being joined beneath it by a for- 

 ward process of the frontal. 

 The mandible contains in the 



young Bird the six bones on each 



side characteristic of Reptiles ; 



the coronary is, however, often 



absent. As a rule the head of 



the quadrate articulates with the 



roof of the tympanic cavity by a 



single facet in Ratita3,by a double 



facet in Carinatoe. The Jiyoid 



always agrees in essential respects with that of the Pigeon ; in the 



Woodpecker the 

 posterior cornua 

 are curved round 

 the head and at- 

 tached to the skull 

 in the neighbour- 

 hood of the right 

 nostril, a very flexi- 

 ble and protrusible 

 tongue being pro- 

 duced. 



The structure of 

 the shoulder-girdle 

 furnishes one of the 



FIG. 10G7.-Skull of Ara (Maca^ (Prom a photograph by ^^ fundamenfcal 



FIG. 1006.- Anas bOSChas (Duck). Ventral 

 view of Skull, a. p.f. anterior palatine 

 foramen ; b. o. basioccipital ; b. pg. basi- 

 pterygoid process ; b. s. basispherioid ; b. t. 

 basi-teinporal ; e. o. exoccipital ; eu. aper- 

 ture of Eustachian tube ;/. in. foramen mag- 

 num; i. c. internal carotid foramen ; j. jugal ; 

 mx maxilla ; mx. p. maxillo-palatine pro- 

 cess ; oc. c. occipital condyle ; pi. palatine ; 

 p. n. posterior nares ; px. premaxilla ; q. 

 quadrate; q.j. quadrato-jugal ; v. vorner ; 

 IX, X, foramen for ninth and tenth nerves ; 

 XII, for twelfth nerve. (From Wieders- 

 heirn's Vertebrata.) 



