AXIAL SKELETON OP THE OSTRICH. 



447 



Occasionally a long delicate ossicle, a seventh sternal rib (though it does not join the 

 sternum), may be developed, extending ventrad and preaxiad from the ventral end of that 

 rib which is postaxial to the one which distally unites with the sixth sternal rib. 



It is applied externally and ventrad to the side of the sixth sternal rib. It is, in fact, 

 the ossified cartilage of a " false " rib. 



The Sternum. 



This is a wide sheet of bone with four margins and two surfaces (figs. 77 & 78). 



Its external (inferior or ventral) surface is convex, but irregularly undulating. 

 Though there is no true keel, yet there is an oval elevated and flattened tract placed 

 in the middle line at the postaxial half of the bone (fig. 11, f). The extreme antero- 

 posterior length of the sternum is to its transverse dimension as about 5 to 3. Medianly 

 and preaxially from the flattened tract a very low ridge may be developed forwards and 

 dorsad. The internal (superior or visceral) surface of the sternum is strongly concave 

 in both directions. At the bottom of the concavity there may be small openings into 

 the substance of the bone. 



THE STERNUM (figs. 77 & 78, \ natural size ; fig. 79, ^ natural size). 



Fig. 79. 



Kg. 77. 



Pig. 78. 



Fig. 77, outer aspect ; fig. 78, inner aspect ; c c, coracoid grooves ; /, flattened tract ; ca ca, costal angles ; 



h; Ix, lateral xiphoid processes ; mx, median xiphoid process. 

 Fig. 79, lateral aspect, showing the five excavations of the pleurosteon separated by five septa, each septum 



with two articular convexities for one of the sternal ribs ; i, one of the ventral articular convexities ; 



s, one of the dorsal articular convexities. 



