AXTAL SKELETON OF 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. 
Tue 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. 77, 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 concaye 
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, 4 natural size). 
Fig. 77, outer aspect; fig. 78, inner aspect; cc, coracoid grooves; f, flattened tract; ca ca, costal angles; 
la la, lateral xiphoid processes ; ma, 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 conyexities ; 
s, one of the dorsal articular convexities. 
