198 THE SKELETON. 



Structure, The ribs consist of cancellous tissue, inclosed in a thin compact 

 layer. 



Development. Each rib, with the exception of the last two, is developed by 

 three centres, one for the shaft, one for the head, and one for the tubercle. The 

 last two have only two centres, that for the tubercle being wanting. Ossification 

 commences in the body of the ribs at a very early period, before its appearance 

 in the vertebra. The epiphysis of the head, which is of a slightly angular 

 shape, and that for the tubercle, of a lenticular form, make their appearance 

 between the sixteenth and twentieth years, and are not united to the rest of the 

 bone until about the twenty-fifth year. 



Attachment of Muscles. The Intercostals, Scalenus Anticus, Scalenus Medius, 

 Scalenus Posticus, Pectoralis Minor, Serratus Magnus, Obliquus Externus, 

 Transversalis. Quadratus Lumborum, Diaphragm, Latissimus Dorsi, Serratus 

 Posticus Superior, Serratus Posticus Inferior, Sacro-lumbalis, Musculus Acces- 

 sorius ad Sacro-lumbalem, Longissimus Dorsi, Cervicalis Ascendens, Levatores 

 Costarum. 



THE COSTAL CAETILAGES. 



The Costal Cartilages (Fig. 136) are white elastic structures, which serve to 

 prolong the ribs, forward to the front of the chest, and contribute very ma- 

 terially to the elasticity of its walls. The first seven are connected with the 

 sternum, the next three with the lower border of the cartilage of the preceding 

 rib. The cartilages of the last two ribs, which have pointed extremities, float 

 freely in the walls of the abdomen. Like the ribs, the costal cartilages vary 

 in their length, breadth, and direction. They increase in length from the first 

 to the seventh, then gradually diminish to the last. They diminish in breadth, 

 as well as the. intervals between them, from the first to the last. They are 

 broad at their attachment to the ribs, and taper towards their sternal extremi- 

 ties, excepting the first two, which are of the same breadth throughout, and the 

 sixth, seventh, and eighth, which are enlarged where their margins are in con- 

 tact. In direction they also vary; the first descends a little, the second is hori- 

 zontal, the third ascends slightly, whilst all the rest follow the course of the 

 ribs for a short extent, and then ascend to the sternum or preceding cartilage. 

 Each costal cartilage presents two surfaces, two borders, and two extremities. 

 The anterior surface is convex, and looks forwards and upwards; that of the 

 first gives attachment to the costo-clavicular ligament; that of the first, second, 

 third, fourth, fifth, and sixth, at their sternal ends, to the Pectoralis Major. The 

 others are covered by, and give partial attachment to, some of the great flat 

 muscles of the abdomen. The posterior surface is concave, and directed back- 

 wards arid downwards, the six or seven inferior ones affording attachment to 

 the Transversalis muscle and the Diaphragm. Of the two borders, the superior 

 is concave ; the inferior convex ; they afford attachment to the Intercostal 

 muscles, the upper border of the sixth giving attachment to the Pectoralis 

 Major muscle. The contiguous borders of the sixth, seventh, and eighth, and 

 sometimes the ninth and tenth costal cartilages present smooth oblong surfaces 

 at the points where they articulate. Of the two extremities, the outer one is 

 continuous with the osseous tissue of the rib to which it belongs. The inner 

 extremity of the first is continuous with the sternum; the six succeeding ones 

 have rounded extremities, which are received into shallow concavities on the 

 lateral margins of the sternum. The inner extremities of the eighth, ninth, and 

 tenth costal cartilages are pointed, and lie in contact with the cartilage above. 

 Those of the eleventh and twelfth are free, and pointed. 



The costal cartilages are most elastic in youth, those of the false ribs being 

 more so than the true. In old age, they become of a deep yellow color. Under 

 certain diseased conditions they are prone to ossify. Dr. Humphry's observa- 

 tions on this subject have led him to regard the ossification of the costal car- 

 tilages as a sign of disease rather than of age. "The ossification takes place 





