THE RIBS, THE STERNUM, AND THE THORAX 141 



The Seventh, the Eighth, the Ninth, and the Tenth (Fig. 98) are 

 the long, slender ribs. They can be distinguished from all that precede 

 them by their length and slenderness and by their small sternal ex- 

 tremity, the terminal surface of which is almost circular. They differ 

 from the eleventh rib in possessing a tubercle articulating with the 

 vertebral transverse process. They have large, pointed heads deeply 

 pitted on the articular surface, short necks with a broad ventral 

 surface, and a dorsal surface which is gradually encroached upon by 

 the tubercular articular surface. The seventh is a tenth longer than 

 the sixth, the eighth not quite an eleventh longer than the seventh, 

 and the ninth not quite a twelfth longer than the eighth. The tenth 

 is of about the same length as the ninth, or a little shorter. 



The Eleventh, Twelfth, and Thirteenth Ribs (Fig. 98) are easily 

 distinguished from all the rest ; they are long, very slender, and have 

 rounded, sloping heads which are not divided by a pit into two facets ; 

 they have no necks and no tubercles. Of the three, the eleventh is the 

 longest and the thirteenth the shortest. 



RECAPITULATION OF THE CHARACTERS OF THE RIBS. 



The head presents few changes throughout the series. In the first 

 rib and the last three the articular surface is single and undivided. 

 In the cephalic ribs the head is more rounded, in the caudal it is 

 pointed, and the dorsal surface slopes in a ventral direction to the 

 point. 



The neck forms a greater angle with the shaft in the cephalic than 

 in the caudal ribs. The cephalo-caudal diameter of the ventral 

 surface is greater in the caudal than in the cephalic ribs. 



The tubercle is present on all the ribs except on the last three. It 

 is highest and most decidedly separated from the neck in the first two. 

 From the second to the eleventh it becomes gradually lower and con- 

 tinuous with the neck. Beginning with the fifth rib, its lateral edge 

 is sharp and prominent and separated from the dorsal-caudal border 

 of the shaft by a depression for ligamentons insertion. The articular 

 surface on the tubercle of the first rib is quadrate, with its cephalo- 

 caudal diameter the larger, and is almost flat from the lateral to the 

 medial edge. On the second rib it is much smaller, transversely oval, 

 and faintly concave. From the second to the eleventh it increases in 

 and turns more to the caudal aspect of the bone. From the first 



