EIBS AND STEENUM. 403 



culated, the remainder being fused with the trans- 

 verse processes of the corresponding vertebrse. 



Of these two, the first is small, but has distinct 

 capitular and tubercular articulations : the second 

 is larger and bears on its posterior border a back- 

 wardly projecting uncinate process, which over- 

 laps the next rib. 



b. The thoracic ribs are five on each side, and slightly 

 increase in size from before backwards : each 

 consists of vertebral and sternal portions, meeting 

 at an angle. 



i. The vertebral portions are flattened and slightly 

 curved bars of bone, which articulate by capi- 

 tular and tubercular surfaces with the centra 

 and transverse processes of their respective 

 vertebrffi. Uncinate processes project back- 

 wards from the posterior borders of all, except 

 the last, or sometimes the last two. 



ii. The sternal portions are slender nearly straight 

 rods of bone, which run forwards and down- 

 wards, from their junctions with the vertebral 

 portions, to the side of the anterior part of 

 the sternum, with which they articulate. 



2 The sternum, or breastbone, is very large, and projects 

 back far beyond the ribs, and over the greater part 

 of the wall of the abdomen. Its ventral surface is 

 raised into a prominent vertical crest or keel, which 

 gives origin to the muscles moving the wings. 



i. The body of the sternum is a somewhat boat- 

 shaped plate of bone : it is wide and stout 

 in front, and perforated on its concave dorsal 

 surface by apertures, through which the air- 

 sacs communicate with the cavity of the bone : 

 it is narrow in the middle portion of its length, 

 but widens somewhat behind. 



ii. The manubrium is a small vertical plate of bone, 



dd2 



