THE BONES OF THE TRUNK 41 



of the second sacral body, and the filum terminale passes on to 

 be attached to the back of the fifth sacral or first coccygeal 

 vertebra. 



B. The Ribs. 



The ribs (costae) are twelve in number at either side, and are 

 arranged in two groups, true or sternal, and false or asternal. The 

 true ribs are those which articulate directly with the sternum by their 

 costal cartilages, and they represent as a rule the first seven at either 

 side. The false ribs are those which have no direct articulation by 

 their costal cartilages with the sternum, and they represent, as a 

 rule, the last five at either side. The last two false ribs, eleventh 

 and twelfth, are called the free or floating ribs, because their costal 

 cartilages stand quite clear of each other and of the tenth. The 

 ribs are elastic, and increase in length from the first to the seventh, 

 whence they decrease to the twelfth. The first is the broadest, 

 and the twelfth the narrowest. Their direction is at first down- 

 wards, outwards, and slightly backwards, then downwards and 

 forwards, and finally inwards. The upper ribs are not so oblique 

 as those lower down, the most oblique being the ninth. With the 

 exception of the first rib, the surfaces of the others are vertically 

 disposed posteriorly, but in front they are sloped downwards and 

 forwards, and this circimistance renders most of them twisted. 



A Typical Rib. — A typical rib presents for consideration a head, 

 neck, tubercle, shaft, and sternal extremity. 



The head (capitulum) forms the posterior or vertebral extremity, 

 and is slightly expanded. It presents an irregularly flat smface 

 and an anterior margin. The surface is marked by two oblique 

 facets, upper and lower, which are separated by a horizontal ridge. 

 The lou-er or primary facet is the larger of the two, and articulates 

 with the large upper or primary demi-facet of the lower of the two 

 thoracic bodies with which the head is connected. The upper 

 facet articulates with the small lower demi-facet on the side of the 

 upper thoracic body, and the intervening ridge gives attachment 

 to the interarticular ligament. The anterior margin gives attach- 

 ment to the anterior costo-central hgament. 



The neck is about i inch long, and is compressed from before 

 backwards. Its anterior surface is smooth and covered by the 

 costal pleura. Its posterior surface, which is rough, faces the anterior 

 surface of the lower thoracic transverse process, or that with which 

 its tubercle articulates, and it gives attachment to the middle 

 costo- transverse or interosseous ligament. Its superior border 

 forms a sharp lip, called the crest, which gives attachment to the 

 superior costo-transverse ligament. Its inferior border may show 

 traces of the subcostal groove. 



The tubercle is situated on the external surface of the rib at the 

 outer extremity of the neck, and presents two divisions, articular 

 and non-articular. The articular division, inferior and slightly 



