THE EIBS, THE STERNUM, AND THE THORAX 143 



to the cephalic-lateral surface ventral to the angle : (a) cephalic 

 division (anterior) on the second to the ninth rib, inclusive ; (b) 

 caudal division (posterior) on the tenth to the thirteenth rib ; the 

 ilio-costal and accessory ilio-costal, to the angles ; the longissimus 

 dorsi, to the dorsal-caudal border, lateral to the tubercle ; the external 

 and internal intercostals, to the borders ; the levatores costarum, to 

 the dorsal-caudal border near the tubercle ; the sternalis, to the lateral 

 surface of the first seven costal cartilages ; the rectus abdominis, to 

 the ventral part of tlje cephalic-lateral surface of the first rib and 

 costal cartilage ; the infracostals, to the medial surface near the 

 angles ; the diaphragm, to the caudal border and medial surface of 

 the last five costal cartilages ; and the triangularis sterni, to the 

 medial surface of the costal cartilages from the second to the eighth, 

 inclusive. 



Blood-Supply. The ribs are supplied with blood by small 

 branches from the intercostal arteries. 



Ossification. Each rib is developed from two principal centres 

 of ossification, one for the shaft and one for the head. The anterior 

 ten ribs each have a small additional centre for the tubercle. 



TABLE. 



Tubercle, with articular facet, present. 

 Head with single facet. 



High tubercle. The shortest rib ; very straight ... 1 

 Head with two facets. 



Sternal end of body with great cephalo-caudal diameter. 



Rib very straight, tubercular facet small .... 2 

 Rib more curved, tubercular facet larger, and cir- 

 cular. Length regularly increases . . . 3, 4, 5, 6 

 Sternal end of body smaller, more rounded. Tuber- 

 cular facet oval, continued inward on neck. 



Length gradually increases 7, 8, 9 



Length the same or decreases 10 



No tubercle with articular facet. 

 Head with single facet. 



Length and degree of curvature decrease gradually . 11, 12, 13 



