RIBS 



57 



is rendered still firmer by the frequent fusion of the vertebrae 

 (p. 48), by the individual ribs often being very broad, as well as 

 by the form and arrangement of the sternum and pectoral arch, 

 which will be treated of later. The last three or four cervical 

 vertebrae may bear comparatively large and movable ribs. The 



FIG. 41. SKELETON OF THE TRUNK OF A FALCON. 



S, scapula ; G, glen old cavity for humerus ; Ca, coracoid, which articulates with 

 the sternum (St) at t ; Fu(Cl), furcula (clavicles) ; Cr, keel of sternum ; 

 V, vertebral, and Sp, sternal, portion of rib ; Un, uncinate process. 



number of ribs which articulate with the sternum varies between 

 two (Dinornis elephantopus) and nine (Cygnus). 

 (Concerning the sacral ribs, see p. 48.) 



Archseopteryx possessed 12-13 pairs of "abdominal ribs" (comp. p. 56). 



Mammals. The cervical ribs here unite completely with the 

 vertebrae, and a vertebrarterial canal is thus formed, as in Croco- 

 diles and Birds. There is considerable variation with regard to the 



