Thorax 



53 



in the lowest spaces in this there is marked prolongation of fibres from the Levator 

 to the next Intercostal, and these fibres, found in the lower spaces, are distinguished 

 as forming " Levatores longiores." 



The non-articular portion of the tubercle is for the posterior costo-transverse 

 ligament : observe that the marking for this ligament is partly on the back and partly 

 on the under surface of the bone, where the line of External Intercostal runs into it. 



Now compare the posterior parts of the remaining ribs, leaving the first rib for 

 separate consideration. The length of the neck is greatest in the middle ribs and 

 decreases above and below. The same is true of the degree of development of the 

 Crista superior, which is largest in the middle and next succeeding members, so that 

 as one ascends or descends the series the area X decreases considerably in actual size 

 and in fact hardly exists in the terminal ribs. There is a corresponding diminution 

 in the size and distinction of the lower area XX, and the ligamentous surface MCT 

 is smaller and comes to look more downward. 



These differences are associated with the degree of development of the superior 

 costo-transverse ligament on the one hand and of the middle ligament on the other. 

 In the last two ribs there is no middle ligament, and the roughened areas lying on the 

 postero-inferior aspects of these bones outside their heads are for the scattered fibres 

 of the posterior ligament further in than in the tenth rib because of the shortness of 

 the last two transverse processes. 



The posterior costo-transverse ligament is stronger on the upper ribs than the 

 lower, and so we find the non-articular part of the tubercle better developed in the 

 former and forming rounded masses. Probably in association with this is the occur- 

 rence of epiphyses for the non-articular portion in the upper five or six ribs, and not 

 in the lower ones : the strongest of the ligaments is on the middle ribs. These liga- 

 ments are strengthened outer bands of the capsule of the tubercle, with which they 

 are continuous, and their development does not appear to be so necessary in the joints 

 that are not in a vertical plane : on examining the articular surfaces of the tubercles 

 we find that those of the upper six are in a vertical plane, while below this they tend 

 to come into a horizontal position, the seventh being transitional : but when the 

 eighth rib is sternal, the seventh tubercular facet is vertical, the eighth is transitional, 

 and the ninth and tenth are less completely horizontal. 



The length of the back of the shaft between the tubercle and angle decreases as we pass up 

 from the eighth rib ; it also lessens as we descend from the eighth, but the decrease is of a different 

 sort. On the eighth rib the length of the part under consideration is about a quarter to a fifth of the 

 whole length of the shaft of the bone, and this proportion still holds, or even shows a very slight 

 increase, in the ribs below this ; but as we ascend we find that it bears an ever-lessening ratio to the 

 length of the shaft, being about one-eighth at the fifth rib, and one ninth at the second. Thus we 

 might say that the decrease below the eighth level is a proportionate one, whereas that above this 

 rib is disproportionate as well as actual. 



The lessening breadth of this area (covered by post-vertebral muscles) upwards 

 is of course dependent on the narrowing in of the muscle mass to bring its outer attach- 

 ments on to the cervical transverse processes, but with it is necessarily associated 

 decrease in size of the muscular areas on the bones, and this is particularly noticeable 

 in the case of those for the Levatores costarum : these muscles become smaller and 

 more blended with the underlying External Intercostals as we ascend, being hard to 

 distinguish on the second rib and absent on the first. 



The secondary markings made by the outer column of the Erector spinae and the 

 aponeurosis can be seen on all the bones except the first : even on the last rib, although 



