GENERAL CHARACTERS OF BIRDS. 649 
the pterygoids usually articulate freely with the basisphenoid, 
the lachrymals may remain free, and there may be a joint in 
the beak at the end of the premaxille. There is but one 
condyle. A membrane bone called the basitemporal covers 
the basisphenoid. There is an interorbital septum formed 
from presphenoid and mesethmoid. The otic bones fuse with 
adjacent bones and with one another about the same time. 
In modern birds there are no teeth, but the jaws are covered 
C g pr. 
Fic. 354.—Position of organs in a bird.—After Selenka. 
n., Nostrils; ¢7., trachea; cv., crop; %., heart; s¢., sternum ; p7., 
proventriculus ; ¢., gizzard; Css ceca : p + pygostyle ; pu. » pelvis; 
&., kidney ; Z., lung. 
by horny sheaths. The premaxille are large, and form most 
of the beak. There is a complete infra-temporal arcade 
formed by a delicate jugal and quadratojugal reaching back 
to the des The supra-temporal arcade is usually 
incomplete, but in some cases a process of the sqguamosal joins 
a postorbital process of the frontal. The lower jaw consists 
on each side of five membrane bones and a cartilage bone— 
the articular—which works on the quadrate. Many of the 
skull bones have a spongy texture, due to cavities filled with 
air from: the nasal and Eustachian tubes. 
