52 Anatomy of Skeleton 



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the membrane supporting the pleura, is attached a'ong the line A separating the two 

 areas and to the lower border of the rib above (AA in Fig. 42, C). 



These two areas are usually apparent on the bone, but the finger will at any rate 

 detect them at once : the line between them can be traced out to become continuous 

 with the line of attachment of the Internal Intercostal muscle. 



The membrane can therefore be looked on as continuing the plane of the Internal 

 Intercostal, and it contains on its deep aspect some aponeurotic fibres continuous with 

 those of the muscle so that in this sense it may be termed the posterior intercostal 

 membrane * just as the anterior membrane continues the plane of the External 

 Intercostal in front. 



The lower border of this part of the bone is a prominent bar that strengthens the 

 curve of the neck, and is continued into the inner lip of the groove on the shaft, to 

 which the Internal Intercostal is attached. 



Behind this prominent rim, the under surface of the neck and neighbouring shaft 

 has a broad groove (XX) which corresponds with the surface X on the upper part of 

 the rib, and is in relation with the same soft iatty tissue : it is bounded behind by the 

 plane of the middle costo-transverse ligament (MCT) and capsule of the costo-trans- 

 verse articulation. Like the upper area X, this lower groove depends for its breadth 

 and depth on the development of the Crista in this case the Crisia inferior -that 

 bounds it behind. The Crista superior affords attachment to the upper costo-trans- 

 verse ligament, of which the more oblique anterior fibres reach the top of the crest 

 and the less oblique posterior ones extend somewhat on to the posterior surface. The 

 External Intercostal is continued as an aponeurotic layer along the dotted line B to 

 the ligament. The markings for all these structures are apparent on the bone. 



On the back of the neck there is a rough ligamentous area in the lower part (MCT) 

 for the middle costo-transverse ligament, extending down to the lower border and 

 out to the costo-transverse capsule. Between this marking and that for the upper 

 ligament the bone is in its " natural state," and lies in front of the inter-transverse 

 tissues without affording attachment to them. 



On the posterior surface of the shaft, between the angle and the tubercle, it is 

 possible to distinguish three areas of markings on the bone. The upper E is for the 

 External Intercostal, as is also the lower and larger one EE, while the intermediate 

 district LC is for the Levator costa? ; these muscles are shown in situ in the lowest 

 drawing, which exhibits their relations to each other and neighbouring structures. 

 E, frequently an even more narrowed area than in the bone figured, is continued out 

 along the top margin of the shaft and inwards into the line B, while EE reaches the 

 tubercle internally and the lower margin of the bone externally. LC is a well-marked 

 part that extends in above the non-articular tubercle, but externally it does not reach 

 the angle, and the smooth piece of bone left in this way is covered by fibres of Acces- 

 sorius, which, with Ilio-costalis and the vertebral aponeurosis, is attached to the bone 

 at the angle and makes the secondary lines here. Tendinous slips of Longissimus 

 reach the bone between LC and EE, usually only in the middle six or seven ribs. 



E and LC are continuous, owing to continuity of the muscular fibres, and some of 

 the ridges of the levator area can be traced occasionally into EE just in front of the 

 tubercle. This last indicates that the fibres of the Levator may occasionally be 

 continued across the bone into the next External Intercostal. 



These possibilities of continuity between the Levator costae and the External 

 Intercostals can be understood by reference to the drawing of the dissected part : 



* I am aware that this is not the usual description, but it appears to tne to accord more truly with what 

 is found on dissection. 



