VERTEBRAL COLUMN 



739 



transverse processes ; the thirteenth and fourteenth have 

 them free and well developed, but not reaching the sternum. 

 Of the thoracic vertebrae, namely, those whose ribs reach 

 the sternum, the anterior three are fused to one another, 

 while the fourth is free. The complex sacral region con- 

 sists of the fifth thoracic (with free ribs reaching the ster- 

 num), five or six lumbars, two sacrals, and five caudals, all 

 fused. Lastly, there are six free caudals ending in a 

 pygostyle or ploughshare bone — a fusion of about four 

 vertebras (cf. coccyx in man). This bone serves as a base 

 for the rectrices. 



6.0c 



E.Oc 



p-jf,^^ 439.— Disarticulation of bird's skull. — After Gadow. 

 Membrane bones shaded. 



BOc basioccipital ; E.Oc, exoccipital ; S.Oc, supraoccipital ; 

 Pa parietal ; Fr., frontal ; Na., nasal ; pm., premaxilla ; M., 

 maxilla ; ]u., jugal ; (?;., quadrato-jugal ; Qu. quadrate ; pe 

 periotic • Sq., squamosal ; A.S., alisphenoid ; B.S., basisphenoid ; 

 OS orbito-sphenoid ; Pr.Sph., presphenoid ; vo.. vomer; 

 ids ' interorbital septum ; E., ethmoid ; Se., nasal septum ; 

 De 'dentary; Sp., splenial ; An., angular; SA., surangular ; 

 Ar!, articular ; MK., Meckel's cartUage. 



A cervical vertebra shows on the anterior surface of the 

 centrum a distinctive curvature, described as saddle-shaped 

 or heterocoelous. It is concave from side to side, convex 

 from above downwards. Posteriorly the curvatures are, of 



course, the reverse. , . 



The ribs have two heads — a capitulum articulatmg with 

 a centrum, a tubercle articulating with a transverse pro- 

 cess. The ventral part of the rib, which reaches the 

 sternum, is called the sternal rib, and is joined at an angle 

 to the dorsal part, which articulates with a vertebra. In 

 Birds the sternal ribs are always bony ; in Mammals they 



