I50 



HUMAN ANATOMY 



exceptionally eight, reach the sternum through their cartilages ; hence they are called 

 sternal ribs,^ as distinguished from the remaining five pairs of asternal ribs.^ Each 

 cartilage of the next three joins that of the rib above it. The last two pairs have 

 the cartilages ending free, and are termed floating ribs. Their complicated curves 

 are best understood by studying them in place. Each rib (with certain exceptions 

 to be detailed later) has an articular surface, the head, at the posterior end ; followed 

 by a narrower neck, succeeded by an articular facet on the tubercle which rests on 

 the transverse process of the vertebra. The first rib has an upper and a lower 

 surface, an outer and an inner border ; the second faces in a direction intermediate 

 to this and the following, which have an outer and an inner surface, an upper and a 

 lower border. They are placed obliquely, the front end being lower than the hind 

 one. The outline of the ribs is irregular, so that their declination is not due wholly 

 to their position, but in part also to their shape. Thus, one in the middle of the 

 series slants a little downward as far as the tubercle, then declines more sharply 

 to a roughness near the tubercle known as the angle, and thence more gradually 

 to the end. The main curve of such a rib is backward, outward, and downward 

 as far as the angle, which marks a rather sudden change of direction, the course 

 changing to one forward, slightly outward, and downward, until, as it reaches the 

 front of the chest, it runs forward, downward, and inward. The external surface 

 is vertical at the back and side and slants slightly upward in front. Bearing the 

 declination of the rib in mind, it is evident that to accomplish this the rib must be 

 twisted on itself, otherwise the upper edge would project in front. 



Fig. 174. 



Articular facets for bodies of vertebrae 



Articular facet on tubercle for transverse process 



Right fifth rib from behind. 



The head ''' is an enlargement at the posterior end and on the outer surface, — i.e. , 

 the one farthest from the cavity of the chest. It has an articular surface at the 

 end facing inward and backward, divided into an upper and a lozver facet, each for 

 the body of a vertebra, by a transverse ridge, whence a ligament passes to the inter- 

 vertebral disk. The lower facet is the larger, and is generally concave ; the upper 

 is nearly plane. The head increases in size to the ninth riband then lessens. 



The neck* is compressed from before backward, smooth in front and rough for 

 ligaments behind. The upper aspect has a sharp border, the crest, ^ for the superior 

 costo-transverse ligament. The neck grows slightly longer in descending the series 

 to the same level. The crest on the top of the neck is most developed in the sixth, 

 seventh, and eighth ribs. 



The tubercle " is an elevation beyond the neck on the posterior surface of the 

 rib, bearing internally a round articular surface facing backward and, in most cases, 

 downward, to rest on the transverse process ; beyond the articular facet is a rough 

 knob for the external costo-transverse ligament. 



The shaft ' is smooth inside, the surface being continuous with that of the neck. 

 The subcostal groove^ for the intercostal vein is best marked in the middle ribs, begin- 

 ning at the tubercle and running forward, growing fainter, along three-quarters of 

 the rib, just under cover of the lower border. The outer surface is rather irregular. 



The angle * at which the shaft changes its direction is marked by a rough line 

 on the posterior surface, some distance beyond the tubercle, receiving muscles from 

 the system of the erector spince. The angle, which is not found in the first rib, is 



' Costae verae. - Costae spuriae. ^Capltulum. ■* Collum. ''Crista colli. '^Tuberculum. "Corpus costae. "Sulcus 

 costalis. -'Angulus costae. 



