SKELETON. 



60 1 



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 consists 

 of the fifth thoracic (with free ribs reaching the sternum), 

 five or six lumbars, two sacrals, and five caudals, all fused. 

 Lastly, there are six free caudals ending in a pygostyle or 



? M.k 



>£.0c. 



Fig. 261. — Disarticulation of bird's skull. — After Gadow. 

 Membrane bones shaded. 



B.Oc, basioccipital ; E.Oc, exoccipit.il; S.Oc, supraoccipital ; 

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

 maxilla ; Ju., jugal ; Qj., quadrato-jugal ; Qu., quadrate ; pe. , 

 periotic ; Sq., squamosal; AS., alisphenoid; B.S., basi- 

 sphenoid ; O.S., orbito-sphenoid ; Pr.Sph., presphenoid ; vo., 

 vomer; io.s., interorbital septum; E., etbmoid ; Se. t nasal 

 septum; De., dentary ; Sp. t splenial ; An., angular; Sa., sur- 

 angular ; Ar., articular ; MK., Meckel's cartilage. 



—a fusion of about four vertebrae (cf. 

 This bone serves as a base for the 



ploughshare bone,- 

 coccyx in man), 

 rectrices. 



A cervical vertebra shows on the anterior surface of the 

 centrum a distinctive curvature, described as saddle-shaped 

 or heteroccelous. 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 articulating with 

 a centrum, a tubercle articulating with a transverse process. 

 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 are usually 

 cartilaginous. On the posterior surface of each of the first 



