234 THE SKELETON. 



strongly marked as far as the inner side of the angle, and gradually becomes lost 

 at the junction of the anterior with the middle third of the bone. The interval 

 between it and the inferior border is deeply grooved, to lodge the intercostal 

 vessels and nerve. At the back part of the bone this groove belongs to the 

 inferior border, but just in front of the angle, where it is deepest and broadest, it 

 corresponds to the internal surface. The superior edge of the groove is rounded ; it 

 serves for the attachment of the Internal intercostal muscle. The inferior edge 



O 



corresponds to the lower margin of the rib and gives attachment to the External 

 intercostal. Within the groove are seen the orifices of numerous small foramina 

 which traverse the wall of the shaft obliquely from before backward. The 

 superior border, thick and rounded, is marked by an external and an internal lip. 

 more distinct behind than in front ; they serve for the attachment of the External 

 and Internal intercostal muscles. The inferior border, thin and sharp, has attached 

 to it the External intercostal muscle. The anterior or sternal extremity is flat- 

 tened, and presents a porous, oval, concave depression, into which the costal 

 cartilage is received. 



Peculiar Ribs. 



The ribs which require especial consideration are five in number viz. the first, 

 second, tenth, eleventh and twelfth. 



The first rib (Fig. 187) is one of the shortest and the most curved of all the ribs ; 

 it is broad and flat, its surfaces looking upward and downward, and its borders 

 inward and outward. The head is of small size, rounded, and presents only a 

 single articular facet for articulation with the body of the first dorsal vertebra. 

 The neck is narrow and rounded. The tuberosity, thick and prominent, rests on 

 the outer border. There is no angle, but in this situation the rib is slightly bent, 

 with the convexity of the bend upward, so that the head of the bone is directed 

 downward. The upper surface of the shaft is marked by two shallow depressions, 

 separated from one another by a small rough surface for the attachment of the 

 Scalenus anticus muscle the groove in front of it transmitting the subclavian 

 vein, that behind it the subclavian artery. Between the groove for the subclavian 

 artery and the tuberosity is a rough surface, for the attachment of the Scalenus 

 medius muscle. The under surface is smooth, and destitute of the groove observed 

 on the other ribs. The outer border is convex, thick, and rounded, and at its 

 posterior part gives attachment to the first serration of the Serratus magnus ; the 

 inner is concave, thin, and sharp, and marked about its centre by the commence- 

 ment of the rough surface for the Scalenus anticus. The anterior extremity is 

 larger and thicker than any of the other ribs. 



The second rib (Fig. 188) is much longer than the first, but bears a very con- 

 siderable resemblance to it in the direction of its curvature. The non-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 upward, to that found in the first rib. The shaft 

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

 ing upward and a little outward. It presents, near the middle, a rough eminence 

 for the attachment of the second and third digitations of the Serratus magnus ; 

 behind and above which is attached the Scalenus posticus. The inner surface, 

 smooth and concave, is directed downward and a little inward ; it presents a short 

 groove toward its posterior part. 



The tenth rib (Fig. 189) has only a single articular facet on its head. 



The eleventh and twelfth ribs (Figs. 190 and 191) 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 inclination downward. 



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



