THE RIBS 



Wi 



ribs are united to one another and to the seventli costal cartilage by ligaments 

 (sometimes by short vertical bars of cartilage), and those of the vertebral ribs end 

 freely. The inner surfaces of the lower six afford attachment to the diaphragm 

 and the transversal is muscle. 



The second, third, fourth, and fifth costal cartilages articulate with tlie sides of 

 the sternum, at a spot corresponding to the junction of two sternebne. The sixtli 

 and seventh (and eighth when this reaches the sternum) are arranged irregularly. 

 As a rule the sixth lies in a recess in the side of the fifth sternebra; the seventh 

 corresponds to the line of junction of the meso- and metasternum; and the eighth 

 articulates with the metasternum (see fig. 112). 



Blood-supply. — The twigs for the costal cartilages are derived from the terminal 

 twigs of the aortic intercostals, and from the internal mammary arteries. 



Development. — At the eighth week of intra-uterine life the ribs are cartilag- 

 inous. About this date an earthy spot appears near the angle of each rib, and 

 spreads with great rapidity along the shaft, and by the fourth month reaches as far 

 as the costal cartilage; the proportion borne 1)y the rib-shaft to the costal cartilage is 

 about the same at this date as in adult life. Whilst the ribs are in a cartilaginous 

 condition, eight of them reach the sternum; even after ossification has taken place, 



Fio. 112. — The Thorax at the Eighth Month. 

 (On the left side eight cartihiges reach the sternum.) 



the costal cartilage of the eighth rib, in many instances, articulates with the sternum 

 as late as the eighth month (fig. 112). This relationship ma}' persist through life, 

 but usually the cartilage retrogresses, and is replaced by ligamentous tissue. About 

 the fifteenth year a secondary centre appears for the head of each rib, and a little 

 later one makes its appearance for the tubercle, except in the tenth, eleventh, and 

 twelfth ribs. The secondary centres fuse with the ribs about the twenty-third 

 year. The rib-shaft increases in length mainh* at its line of junction Avith the 

 costal cartilage. 



Variations in the y^umber and Shape of the Ribs 



The ribs may be increased in number by addition either at the cervical or lumbar end of the 

 series, but it is extremely rare to find an additional rib or pair of ribs in both the cervical and 

 lumbar regions in the same subject. 



Cervical ribs are fairly common ; as a rale thej' are of small size and rarely extend more 

 than a few millimetres beyond the extremity t)f the transverse process. Occasionally they 

 exceed such insignificant proportions and reach as far as the sternum : between these two ex- 

 tremes many varieties occur. As a rule, the existence of a cervical rib is not detected until the 

 skeleton is macerated ; hence we know little of the correlated arrangement of soit parts. In 

 one fortunate case Turner was able to make a thorough dissection of a specimen in which a 

 comi»lete cei-vical rib existed. Its head articulated with the body of the seventh cervical ver- 

 tebra and had a stellate ligament. The tuberele was well developed, and articulated with the 

 transverse process. The costal cartilage blended with that of the first thoracic rib, and gave 



