XIII 



PHYLUM CHORDATA 



429 



inxiL 



Bird. When the mandible is 

 depressed, the contraction of the 

 digastric muscle causes a for- 

 ward movement of the lower end 

 of the quadrate, which pushes 

 forwards the maxillo-iuijal bar 

 and the palatines and ])tery- 

 goids, the latter sliding upon 

 the rostrum. Both the maxilla; 

 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 lacryraal 

 being joined beneath it by a for- 

 ward process of the frontal. 



The mandiUe 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 (^^^'^f^^/'^i^e articulates with the 

 roof of the tympanic cavity by a 

 single fVicet in Ratita3,by a double 

 facet in Carinatw. The hyoid 



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



Woodpecker 



Fig. 10ii().- Anas b08Clias{ Duck). Ventral 

 view of Skull. (i. p. f. anterior palatine 

 foramen ; b. o. ba.sioceipital ; h. pri. basi- 

 pterygoid iJrocess ; 6. -f. basispheiioid ; 6. t. 

 basi-tenipoi-al ; e. o. exoccipital ; eu. aper- 

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

 num; '. r. internal carotid foramen ; J. jugal ; 

 »(./■ maxilla ; m.v. ji. maxillo-palatine pro- 

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

 JI. n. posterior nares ; /)./■. premaxilla ; 'y. 

 quadrate; q.j. quadrato-jugal ; r. vomer; 

 IX, X, foramen for nirith and tenth nerves ; 

 A7/, for twelfth nerve. (From Wicdcrs- 

 heim's Verlebra.ta.) 



Fig. 1007.— Skull 



of Ara (Macaw). (From 

 A. Hamilton.) 



a photograph by 



the 

 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 

 most fundamental 



