2 30 ZOOTOMY. 



from near the middle line immediately ventral to the 

 rostrum, outwards and slightly backwards to the costal 

 processes. 



87. The keel or carina sterni, a median ventral plate 

 of bone, running along the whole middle ventral line of the 

 corpus sterni from the base of the rostrum to the middle 

 xiphoid process : its depth is about equal to the breadth of 

 the body of the sternum. 



88. In a bird about three or four weeks old, the sternum is formed of 

 three distinct bones : one median, the lophosteon, ossifying the central 

 part of the body and the keel, and an external pair, the pleurostea, 

 forming the lateral parts. ^ 



XII. In the shoulder girdle note : 

 89. The coracoids, paired rod-Hke bones, articulating by 

 their laterally expanded ventral ends with the coracoid grooves 

 of the sternum, and passing forwards, upwards, and slightly 

 outwards : each coracoid is somewhat depressed from above 

 downwards, and presents at its upperend a thickened head : 

 immediately below the head on the dorsal side of the bone 

 are two facets for the articulation of the scapula : on the 

 inner border at about the same level is a facet for the head 

 of the furcula (§ 91), and on the outer border a large 

 demi-facet which forms the anterior half of the glenoid 

 cavity for the articulation of the wing: towards the inner 

 border of the upper end of the coracoid is a groove passing 

 into a canal which opens on the upper surface of the head 

 between the scapular facets, and serves for the passage of the 

 tendon of the subclavius (§ 326, p. 252) : diis cavity, being 

 completed by the articulation of the scapula (§ 90) and the 

 furcula (§ 91) with the coracoid, is known as the foramen 

 triosseum. 



' In the young chick a separate pair of ossifications, the metostea, 

 2i\es rise to the internal and external xiphoid processes. 



