THE COSTAL CARTILAGES 127 



have no necks or tubercles, and are pointed at their anterior ends. The eleventh 

 has a slight angle and a shallow costal groove. The twelfth has neither; it is much 

 shorter than the eleventh, and its head is inclined slightly downward. Sometimes 

 the twelfth rib is even shorter than the first. 



Structure. The ribs consist of highly vascular cancellous tissue, enclosed in a thin layer of 

 compact bone. 



Ossification. Each rib, with the exception of the last two, is ossified from four centers; a 

 primary center for the body, and three epiphysial centers, one for the head and one each for the 

 articular and non-articular parts of the tubercle. The eleventh and twelfth ribs have each only 

 two centers, those for the tubercles being wanting. Ossification begins near the angle toward the 

 end of the second month of fetal life, and is seen first in the sixth and seventh ribs. The epiphyses 

 for the head and tubercle make their appearance between the sixteenth and twentieth years, and 

 are united to the body about the twenty-fifth year. Fawcett 1 states that " in all probability there 

 is usually no epiphysis on the non-articular part of the tuberosity below the sixth or seventh rib. 



The Costal Cartilages (Cartilagines Costales). 



The costal cartilages (Fig. 115) are bars of hyaline cartilage which serve to 

 prolong the ribs forward and contribute very materially to the elasticity of the 

 walls of the thorax. The first seven pairs are connected with the sternum; the 

 next three are each articulated with the lower border of the cartilage of the pre- 

 ceding rib; the last two have pointed extremities, which end in the wall 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 

 decrease to the twelfth. Their breadth, as well as that of the intervals between 

 them, diminishes from the first to the last. They are broad at their attachments 

 to the ribs, and taper toward their sternal extremities, excepting the first two, 

 vhich are of the same breadth throughout, and the sixth, seventh, and eighth, 

 v.-hich are enlarged where their margins are in contact. They also vary in direc- 

 tion: the first descends a little, the second is horizontal, the third ascends slightly, 

 while the others are angular, following the course of the ribs for a short distance, 

 a nd then ascending to the sternum or preceding cartilage. Each costal cartilage 

 presents two surfaces, two borders, and two extremities. 



Surfaces. The anterior surface is convex, and looks forward and upward: that 

 of the first gives attachment to the costoclavicular ligament and the Subclavius 

 muscle; those of the first six or seven at their sternal ends, to the Pectoralis major. 

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

 of the abdomen. The posterior surface is concave, and directed backward and 

 downward; that of the first gives attachment to the Sternothyroideus, those of 

 ihe third to the sixth inclusive to the Transversus thoracis, and the six or seven 



H'nferior ones to the Transversus abdominis and the diaphragm. 

 Borders. Of the two borders the superior is concave, the inferior convex; they 

 afford attachment to the Intercostales interni: the upper border of the sixth gives 

 attachment also to the Pectoralis major. The inferior borders of the sixth, seventh, 

 eighth, and ninth cartilages present heel-like projections at the points of greatest 

 convexity. These projections carry smooth oblong facets which articulate respec- 

 tively with facets on slight projections from the upper borders of the seventh, 

 eighth, ninth, and tenth cartilages. 



Extremities. The lateral end of each cartilage is continuous with the osseous 

 tissue of the rib to which it belongs. The medial end of the first is continuous 

 with the sternum; the medial ends of the six succeeding ones are rounded and are 

 received into shallow concavities on the lateral margins of the sternum. The 

 medial ends of the eighth, ninth, and tenth costal cartilages are pointed, and are 

 connected each with the cartilage immediately above. Those of the eleventh and 

 twelfth are pointed and free. In old age the costal cartilages are prone to undergo 

 superficial ossification. 





1 Journal of Anatomy and Physiology, vol. xlv. 



