THE RIBS 109 



to ossify about the sixth month of foetal life. About this time a single centre appears 

 in the manubrium; at birth this is well developed. Two centres placed vertically 

 have also been recorded. Secondary epiphyses have been described in connexion with 

 the clavicular facets ; these do not unite with the rest of the manubrium till adult life 

 is reached. The body, formed by the fusion of four segments, is ossified from independent 

 centres, either single or double, for each segment. These appear the highest as early as 

 the sixth month of intrauterine life in some cases even before the manubrium has begun 

 to ossify (Lambertz), the lowest towards the end of full term. The common arrange- 

 ment met with at birth is a single centre for the first, and double centres for each 

 of the succeeding segments. Union between these segments occurs rather irregu- 

 larly, and is liable to much variation. The fourth unites with the third segment in 

 early childhood, the third with the second about puberty, whilst the fusion of the second 

 with the first segment may not be complete till the twentieth or twenty-fifth year. 



The xiphoid process usually ossifies from a single centre, which may appear as early as 

 the third year, though often very much later. The xiphoid process usually unites with the 

 body about forty or fifty, and in exceptional cases osseous union between the body and 

 manubrium may occur in advanced life. 



According to Paterson the manubrium or presternum is developed in association with 

 the shoulder girdle and becomes only secondarily associated with the ventrally growing ribs. 



COSTJE RIBS. 



The ribs, of which there are twelve pairs, form a series of curved osseous 

 bands which support the thoracic wall; posteriorly they articulate with the 

 thoracic vertebrae; anteriorly, each rib is provided with a costal cartilage. The 

 first seven ribs articulate with the sternum by means of their cartilages, and 

 are termed the costse verse, true or vertebro-sternal ribs. The lower five ribs are 

 not so supported, and are described as the costse spurise (false ribs). Of these the 

 eighth, ninth, and tenth are united by their cartilages to the cartilage of the 

 seventh rib, and are called the vertebro-chondral ribs, whilst the last two ribs are 

 free at their anterior extremities, and are named the floating or vertebral ribs. 



A typical rib consists of a capitulum costse (head), a collum costse (neck), a 

 tuberculum costse (tubercle), and a corpus costse (shaft), on which, near its posterior 

 end, is the angulus costse (angle). 



The head, placed on the posterior or vertebral end of the bone, is somewhat 

 expanded. Medially, its articular surface is wedge-shaped and divided into two 

 parts, a superior and inferior, by a ridge or crest (crista capituli), to which the inter- 

 articular ligament of the head of the rib is attached. Of these two facets the 

 inferior is usually the larger, and articulates with the superior facet on the body 

 of the vertebra in numerical correspondence with it, whilst the superior facet is for 

 the corresponding area on the inferior part of the body of the vertebra above. 

 The head is supported by a more or less constricted bar of bone, the neck, which 

 lies anterior to the transverse process of the lower of the two vertebrae with which 

 the rib articulates, and thus assists in the formation of the costo-transverse cleft. 

 The neck is continuous with the shaft laterally, at which point there is a well-marked 

 tubercle on its posterior surface. The anterior surface of the neck is smooth ; its 

 posterior aspect is rough, and pierced by numerous small holes for vessels. Here is 

 attached the ligament of the neck of the rib. Not uncommonly the superior border 

 of the neck is lipped and ridged (crista colli costse), especially in the case of the sixth, 

 seventh, and eighth ribs, and affords attachment to the anterior and posterior costo- 

 transverse ligament. The inferior border is continuous with the costal groove laterally. 



The tubercle consists of an articular and a non-articular part ; the former is 

 medial and inferior to the latter. Its articular surface, of rounded or oval shape, 

 is directed downwards, posteriorly, and a little medially, and rests upon a facet on the 

 transverse process of the vertebra in numerical correspondence with the rib. The 

 non-articular part, most prominent in the upper ribs, has the fibres of the ligament 

 of the tubercle of the rib attached to it. It is usually separated from the superior 

 border of the neck and shaft by a groove, in which lies the lateral division of the 

 posterior ramus of a thoracic nerve. 



The body is thin, flattened, and band-like. Its length varies much ; 



