608 CHORD ATA. 



are hard horny sheaths covering the jaws which are frequently car- 

 ried back on the outside into a softer cere (fig. 634, WH). 



The cranium consists of four occipitals, a basi- and a presphenoid; above, 

 the parietals and frontals ; and on the sides prootics, alisphenoids and 

 orbitosphenoids, while the broad squamosals also enter its wall. The large 

 mesethmoid lies in the interorbital septum ; the nasal cavity is roofed by 

 the nasals, and beside them are the lachrymals. The quadrate articulates 

 with the squamosal, and from it extend forward internally the pterygoid, 

 palatine, and vomer ; externally a zygomatic arch of quadratojugal and 

 jugal to the maxillaries and premaxillaries. The maxillaries are hinged 

 in the ethmoidal region, so that in opening the mouth there is besides the 

 depression of the lower jaw an upward motion of the upper jaw. 



The pneumaticity of the bones is an important feature of the 

 skeleton. In place of marrow and bony tissue, the inside of the 

 bones in strong flying birds is more or less completely occupied by 

 air spaces, around which, as a sheath, is the compact bone. This 

 gives the greatest possible lightness and strength to the skeleton. 

 In Buceros and Palamedea all of the bones are pneumatic; in 

 others (Pelecanus, Sula, Tachy petes, etc.) only the phalanges of 

 the toes contain marrow, while in the penguin and Apteryx, as in 

 mammals, air spaces occur only in some of the cranial bones. 



The air spaces of the bones are in part (skull) connected with 

 the nose and tympanum, but most of them, by means of the air 

 sacs, communicate with the lungs. The long trachea forks at its 

 lower end into two bronchi. At its upper end is a larynx, as in 

 other vertebrates, but this is not vocal ; the notes of birds are pro- 

 duced by the syrinx, which lies at the division of trachea into 

 bronchi. It is usually formed of both trachea and bronchi, but 

 more rarely of either trachea or bronchi alone. Its vocal cords 

 are regulated by special muscles, which in the singing birds have 

 a complicated arrangement. The relatively 

 small lungs send out from their surface air 

 sacs, especially well seen in embryos (fig. 638, 

 , 3 1-5). These later become large, thin-walled 

 spaces, easily torn away in dissection, leaving 

 * large openings on the surface of the lungs 

 (fig. 639, 1-5). Usually five pairs of these 

 air sacs are present, largely in the coelom, 

 (Aft g er 8a leienk a a.) chic tr; but extending in between the muscles (breast 

 trachea; 1-5, lung sacs! and ax ii] ar y region), and also into the bones. 



The spongy lungs lie on either side of the vertebral column and are 



