98 THE SKELETON 



THE RIBS AND STERNUM 



The ribs. — These fonn a series of narrow ilattened hones, twenty-four in 

 nuniher, arranged in tweh'e ]iairs, extending from the sides of the thoraeie vertehne 

 towards the median line on the anterior as})ect of the trunk. 



The lirst seven pairs are termed true ribs, lieeause their anterior ends are 

 direetly eoimected by means of cartihige with the sides of the sternum. The lower 

 five — false ribs — are classed into two sets: the eighth, ninth, and tenth are 

 connected together l:)y their costal cartilages; the eleventh and twelftli have their 

 anterior extremities free, and are called in consequence floating ribs. Thus the 

 first seven are, vertebro-sternal ; the eighth, ninth, and tenth, vertebro-chon- 

 dral ; the eleventh and twelftli. vertebral ribs. 



The rihs increase in length from the first to the seventh, and then decrease from 

 this rib to the twelfth. 



In l)readth they increase frc^m liehind forwards; the greatest In-eadth of a rih is 

 at its junction with the costal cartilage. The two or three upper ribs form nearly 

 a right angle with the spine, Init the succeeding set curve obliquely downwards. 

 The ol)liquity is greatest at the ninth, and then decreases in the ribs below. 



Typical characters of a rib. — The seventh is regarded as the most typical ril). 

 It presents a vertebral extremity or head ; a narrow portion or neck ; a sternal 

 end; and an intermediate portion, the shaft. 



The head has two facets divided by a horizontal crest. The crest is connected 

 by an interosseous ligament with an intervertebral disc; the facets articulate with the 

 demi-facets on the sides of the liodies of two vertebrae. As a rule, the lower facet 

 is the larger, and articulates with the thoracic vertebra, to which the rib corresponds 

 in num])er. This is the primary facet, and is the one represented in those ribs 

 which possess only a single facet on the rib-head. The anterior margin is lipped 

 for the costo- vertebral (stellate) ligament. 



The neck is that portion of the ril^ extending from the head to the tubercle. 

 The posterior surface of the neck is in relation with the transverse process of the 

 lower vertebra with which the head articulates; it forms the anterior boundary of 

 the costo-transverse foramen, and is rough where it gives attachment to the 

 middle costo-vertel)ral ligament. The superior liorder of the neck is continuous 

 with the corresponding border of the shaft, and at the point where the neck ends 

 this border is produced so as to form a crest (crista colli superior) for tlie anterior 

 costo-transverse ligament. The inferior border of the neck is continuous with the 

 ridge of the subcostal groove. This difference in the relation of the neck to the 

 upper and lower l)i)rders of the ril) shaft is useful in determining to which side a 

 rib belongs. 



The tubercle consists of an upper part, rough for the posterior costo-transverse 

 (rhomboid) ligament, and a lower faceted portion for articulation with the ti}) of 

 the transverse process. The tubercle projects below the lower edge of the ril) to 

 form a crest (crista colli inferior), marking the beginning of the subcostal groove. 



The shaft has two surfaces and two borders. It is strongly curved. At first 

 the curve is in the same i)lane as the neck, but it quickly turns forwards at a si)ot 

 on the i)osterior surface of the shaft known as the angle," where it gives attachment 

 to the sacro-luiithalis muscle and some of its subdivisions. The rib has also a second 

 or ui)ward curve beginning at the angle. These curves are expressed by describing 

 the main curve as disposed around a vertical, and the second or ujiward curve 

 around a transverse axis. 



When a rib, except the first and twelfth, is laid Avith its lower edge uj^on the 

 table, the rib-head rises and the rib touches the table at two places, viz. at tlie 

 sternal end, and in the neighbourhood of the angle. 



The external surface of the ril) is convex, and gives attachment to muscles. 

 Near its anterior extremity it forms a somewhat abrui)t curve, indicated by a ridge 

 on the bone, Avhicli gives attaclunent to the serratm magnus nmscle, and is some- 

 times called the anterior angle. 



