THE RIBS. 129 



they again diminish to the twelfth. In breadth they decrease from above down- 

 ward ; in the upper ten the greatest breadth is at the sternal extremity. 



Common Characters of the Ribs (Fig. 85). A rib from the middle of the 

 series should be taken in order to study the common characters of the ribs. 



Each rib presents two extremities, a posterior or vertebral, an anterior or ster- 

 nal, and an intervening portion the body or shaft. 



The posterior or vertebral extremity presents for examination a head, neck, 

 and tuberosity. The head (Fig. 86) is marked by a kidney-shaped articular sur- 

 face, divided by a horizontal ridge into two facets for articulation with the costal 

 cavity formed by the junction of the bodies of two contiguous dorsal vertebrae ; 

 the upper facet is small, the inferior one of larger size ; the ridge separating them 

 serves for the attachment of the interarticular ligament. The neck is that flat- 

 tened portion of the rib which extends outward from the head ; it is about an 

 inch long, and is placed in front of the transverse process of the lower of the two 

 vertebrae with which the head articulates. Its anterior surface is flat and smooth, 

 its posterior rough for the attachment of the middle costo-transverse ligament, 

 and perforated by numerous foramina, the direction of which is less constant than 

 those found on the inner surface of the shaft. Of its two borders the superior 

 presents a rough crest for the attachment of the anterior costo-transverse ligament ; 

 its inferior border is rounded. On the posterior surface of the neck, just where it 



Facet for body of upper dorsal vertebra. 

 'Ridge for interarticular ligament. 

 Facet for body of lower dorsal vertebra. 



Articular part of tuberosity. 



Non-articular part of tuberosity. 

 FIG. 86. Vertebral extremity of ^ rib. External surface. 



joins the shaft, and nearer the lower than the upper border, is an eminence the 

 tuberosity, or tubercle ; it consists of an articular and a non-articular portion. 

 The articular portion, the more internal and inferior of the two, presents a small, 

 oval surface for articulation with the extremity of the transverse process of the 

 lower of the two vertebrae to which the head is connected. The non-articular 

 portion is a rough elevation, which affords attachment to the posterior costo- 

 transverse ligament. The tubercle is much more prominent in the upper than in 

 the lower ribs. 



The shaft is thin and flat, so as to present two surfaces, an external and an 

 internal, and two borders, a superior and an inferior. The external surface is 

 convex, smooth and marked at its back part, a little in front of the tuberosity, by 

 a prominent line, directed obliquely from above downward and outward; this 

 gives attachment to a tendon of the Ilio-costalis muscle or of one of its accessory 

 portions, and is called the angle. At this point the rib is bent in two directions. 

 If the rib is laid upon its lower border, it will be seen that the portion of the shaft 

 in front of the angle rests upon this border, while the portion of the shaft behind 

 the angle is bent inward and at the same time tilted upward. The interval 

 between the angle and the tuberosity increases gradually from the second to the 

 tenth rib. The portion of bone between these two parts is rounded, rough, and 

 irregular, and serves for the attachment of the Longissimus dorsi muscle. The 

 portion of bone between the tubercle and sternal extremity is also slightly twisted 

 upon its own axis, the external surface looking downward behind the angle, a little 

 upward in front of it. This surface presents, toward its sternal extremity, an 

 oblique line, the anterior angle. The internal surface is concave, smooth, directed 

 a little upward behind the angle, a little downward in front of it. This surface 



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