THE COSTAL CARTILAGES 165 



articular portion of the tuberosity is occasionally only slightly marked. The angle 

 is slight and situated close to the tuberosity, and the shaft is not twisted, so that 

 both ends touch any plane surface upon which it may be laid; but there is a similar 

 though slighter bend, with its convexity upv ard, to that found in the first rib. The 

 shaft is not horizontal, like that of the first rib, its outer surface, which is convex, 

 looking upward and a little outward. It presents, near the middle, a rough emi- 

 nence (tuberositas costae II), for the attachment of part of the first and all of the 

 second digitations of the Serratus magnus; behind and above which is attached 

 the Scalenus posticus. The inner surface, smooth and concave, is directed down- 

 ward and a little inward; it presents a short groove toward its posterior part. 



Tenth Rib. The tenth rib (Fig 133) has only a single articular facet on its head. 



Eleventh and Twelfth Ribs. The eleventh and twelfth ribs (Figs. 134 and 

 135) have each a single articular facet on the head, which is of rather large size; 

 they have no neck or tuberosity, and are pointed at the extremity. The eleventh 

 has a slight angle and a shallow groove on the lower border. The twelfth has 

 neither, and is much shorter than the eleventh, and the head has a slight inclina- 

 tion downward. Sometimes the twelfth rib is even shorter than the first. 



Structure. The ribs consist of cancellous tissue enclosed in a thin layer of compact bone. 



Development. Each rib, with the exception of the last two, is developed from three centres, 

 one for the shaft near the angle, one for the head, and one for the tubercle. The last two ribs 

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

 shaft of the ribs between the ninth and eleventh weeks before its appearance in the vertebrae. 

 The epiphysis of the head, which is of 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. To nineteen the Intercostales externi et interni, Scalenus 

 anticus, Scalenus medius, Scalenus posticus, Pectoralis minor, Serratus magnus, Obliquus 

 externus abdominis, Quadratus lumborum, Diaphragm, Latissimus dorsi, Serratus posticus 

 superior, Serratus posticus inferior, Iliocostalis, Musculus accessorius ad iliocostalem, Lon- 

 gissimus dorsi, Cervicalis ascendens, Levatores costarum, and Infracostales. 



The Costal Cartilages. 



The costal cartilage (cartilago costalis) (Fig. 122) is white, hyaline cartilage. The 

 cartilages serve to prolong the ribs forward to the front of the thorax, and they 

 contribute very materially to the elasticity of its walls. The first seven are con- 

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

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

 which terminate in free ends 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 toward their sternal 

 extremities, 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 contact. In direction they also vary; the first descends a little, the second is 

 horizontal, the third ascends slightly, while 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. 



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; that of the second, third, fourth, fifth, and sixth, at their sternal ends, 

 to the Pectoralis major. 1 The others are covered by, and give partial attachment 

 to, some of the great flat muscles of the abdomen. The posterior surface is con- 



1 The first and seventh also, occasionally, give origin to the same muscle. 



