130 



THE SKELETON 



be noted, however, that the distance between the tubercle and the angle in this rib is greater 

 than in the ribs above. Speaking generally, the distance between these points increases from 

 above downward — a disposition which is useful in at once determining if any given rib belongs 

 to the upper or lower end of the series. 



The eleventh rib is peculiar in that it has a single facet on the head, a feebly marked angle 

 some distance from the head, a shallow costal groove, no tubercle, and no neck. The tubercle 

 is sometimes represented by a slight elevation or roughness without any articular facet. The 

 anterior extremity is pointed. 



The twelfth rib has a large head furnished with one facet for articulation with the root 

 (pedicle) of the twelfth thoracic vertebra. The shaft is narrow and extremely vai'iable in length 

 (3 to 20 cm.). It is usually somewhat longer than the first rib, but it may be shorter. There 

 is no tubercle, no angle, no neck, no costal groove. The anterior extremity is pointed. Poste- 

 riorly, the upper border is smooth and somewhat rounded; the lower border is sharp and 

 rough. 



The costal cartilages are bars of hyaline cartilage attached to the anterior extremities of the 

 ribs, and may be regarded as representing unossified epiphyses. Like the shaft of a rib, each 

 cartilage has an outer and inner surface. The outer surfaces give origin and insertion to large 

 muscles, and the inner surfaces, from the second to the sixth inclusive, are in relation with the 

 transversus thoracis (triangularis sterni). The upper and lower borders serve for the attachment 

 of the internal intercostal muscles. The upper seven cartilages, and occasionally the eighth, 

 are connected with the sternum. Of these, the first fuses with the manubrium sterni and the 

 remaining six are received into small articular concavities, and retained by means of ligaments. 



Fig. 156. — The Vertebral Ends of Tenth, Eleventh, and Twelfth Ribs. 



Angle 



Single facet Csometimes 

 two facets are present) 



Single facet (this rib has 

 an angle, but no tuber- 

 osity and no neck) 



Single facet ("this rib has 

 neither tuberosity, angle, 

 nor neck) 



xn 



The cartilages of the vertebro-chondral ribs are united to one another and to the seventh costal 

 cartilage by ligaments (sometimes by short vertical bars of cartilage), while those of the verte- 

 bral ribs form no such attachment, but lie between the abdominal muscles. The inner surfaces 

 of the lower six costal cartilages afford attachment to the diaphragm and the transversalis muscle. 



Each of the .second, third, fourth, and fifth costal cartilages articulates with the side of the 

 sternum, at a point corresponding to the junction of two sternebra^i The sixth and seventh 

 (and eighth when this reaches the sternum) are arranged irregularly. As a rule, the sixth lies 

 in a recess at 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 figs. 158, 

 161). 



Blood-supply. — The costal cartilages derive their blood-supply from the terminal twigs of 

 the aortic intcrcostals and from the internal mammary arteries. 



Ossification. — At the eighth week of intra-utorine life the ribs are cartilaginous. About 

 this date a nucleus 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. At this date the length of 

 rib-shaft bears the same proportion to that of the costal cartilage as in adult life. Whilst the 

 ribs are in a cartilaginous condition, the first eight rench to the side of the sternum, and even 

 after ossification has taken place, the costal cartilage of the eighth rib, in many instances, retains 

 its articulation with the stennun up to as late as the eighth month (fig. 158). This relationship 

 may 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 eleventh and twelfth rib.s. 

 Frequently epiphyses are developed on both parts of the tubercle (see figs. 159 and 160). The 



