32 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 24. — The first rib of the right side seen from above and from the side (f). 



Fig. 25. — The second rib of the right side seen from above and from the side (f). 



Fig. 26. — The posterior extremity of the seventh rib of the right side seen from behind and partly from 



below (§). 



Fig. 27. — The seventh rib of the left side seen from behind (§). 



Fig. 28. — The seventh rib of the left side seen from within (§). 



THE RIBS. 



The ribs {costcE) belong to that limited group of skeletal segments which remain partly 

 cartilaginous throughout life. Attention may first be directed to the bony ribs, of which there 

 are twelve pairs (Figs. 24 and 30), corresponding to the number of the thoracic vertebrae. 



The ribs are not tubular bones, but flat bony strips of considerable length. Those in the 

 middle of the series are quite uniform in shape, only the upper and lower ones showing certain 

 deviations. 



In a typical rib the following parts may be recognized: 



1. The head, the posterior (vertebral) slightly thickened end. It presents an articular 

 surface (Fig. 27), which is opposed to the bodies of the vertebrae, and is marked by a median 

 ridge, the crest, which divides it into an upper and a lower articular surface for the two vertebral 

 bodies with which it articulates. 



2. The neck, a constriction situated external to the head (Fig. 28). Its upper margin is 

 formed by a ridge, crest 0} the neck, which is separated from the body of the rib by a rough emi- 

 nence, the tubercle, presenting a facet for articulation with the transverse process of a thoracic 

 vertebra. 



3. The body, the longest part of the rib. This is placed vertically and is distinctly flattened 

 from without inward, so that an external and an internal surface may be recognized; near the 

 tubercle it presents a rough surface, the angle of the rib (Fig. 28). The rib is at first directed 

 outward, backward, and downward, but at the costal angle it turns upon itself and passes for- 

 ward. On the inner surface of the lower margin of the body there is a groove, the costal groove 

 (Fig. 28), which gradually becomes shallower as it approaches the anterior costal extremity, 

 and causes the lower margin of the rib to be sharp while the upper one is more rounded. At 

 its anterior extremity, the body of the rib presents a roughened, somewhat shallow surface for 

 the reception of the costal cartilage. 



The typical ribs are the third to the tenth, the first two and the last two exhibiting certain 

 peculiarities. The first rib (Fig. 24) is short and broad; it is not placed vertically but almost 

 horizontally, so that an upper and a lower surface may be recognized in its posterior portion, 

 and an upper or outer and a lower or inner surface in its anterior portion. Its head has no 

 crest, since the first rib usually articulates with the body of the first thoracic vertebra only 

 and not with two adjacent vertebrae, and its angle coincides with the tubercle. Near the 

 anterior extremity of its body there is a low rough protuberance, known as the scalene tubercU 

 or tubercle 0] Lisjranc, for the insertion of the scalenus anticus muscle, and behind this tubercle 



