fi08 CHORD AT A. 



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

 ried back on the outside into a softer cere (fig. 034, WH). 



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

 the pai'ietals and frontals ; and on the sides prootics, alisphenoids and 

 orbitospbenoids, 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 jjnoumaticity of the bones is an important feattire of tlie 

 slieletou. In phtce of marrow and bony tissue, the inside of the 

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

 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 PaJamedea all of the I)ones are pneumatic; in 

 others {Pelecamis, Sula, Tacliypetes, etc.) only the jjhalanges of 

 the toes contain marrow, while in the p>engnin and Apteryx, as in 

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



The air spaces of the bones are in -p&Ti (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 npjier end is a larynx, as iu 



other vertebrates, but this is not vocal ; the notes of birds are ju'o- 



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 



U -51 '» _+_ *'' small lungs send out from their surface air 



^ 3 sacs, especially well seen in embryos (fig. C38, 



) " ''^{^^ 3 1-5). These later become large, thin-walled 



' ^, ''"V^' t spaces, easily torn away in dissection, leaving 



,4^/ "^"^M. r I'^i'gc openings on the surface of the lungs 



'^ (fig. G30, i-.:<). Usuallv five pairs of these 



Fig. fas. — Development •„ „,,, „ j. i" i • j.i i 



"f traoiiea, lungs, and IU sa("s are iiresent, largely 111 the cceiom, 

 (Afto/'''seimka.f"'(r; 1^"^ extending in between the muscles (breast 

 ti-achea; i-,-, lung sacs! ^^^1 axillary region), and also into the bones. 



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



