Thorax 



55 



Its inner margin affords attachment to Sibson's fascia (and the Scalenus pleuralis 

 if present), and its outer margin has a marking about half-way round for the origin of 

 Serratus magnus : behind this the Scalenus posticus crosses the margin, just in front 

 of the tubercle, and passes deep to the digitation of the Serratus to reach the second 

 rib. In front of the origin of Serratus magnus the margin is in relation to the costo- 

 coracoid membrane and Pectoralis major. 



The lower surface of the first rib is nearly altogether pleural in its front hah*, the 

 intercostal muscles being only attached along the outer margin. The various areas 

 are shown in Fig. 44. 



The narrowing of the pleural surface to a line opposite and beyond the angle in 

 the second rib has been noticed, as has also the smalhiess of the insertion of Levator 

 costae and its continuity with the upper external intercostal. Pass the finger along 

 the most prominent convexity of the bone just outside the angle and an irregularity 

 will be detected which marks the attachment of Serratus posticus superior to the rib. 

 Above and in front of this, near the upper margin, is another impression for Scalenus 

 posticus, and these two muscles lie deep to Serratus magnus. The origin of Serratus 

 magnus is from the prominent boss on the shaft, and immediately in front of this is a 

 flatness, or even a shallow concavity, about an inch and a half broad, on which the 

 Pectoralis minor rests when the arm is hanging by the side. These relations are shown 

 in Fig. 43. 



The inner surface of the last rib looks in an upward and inward direction in its 

 whole extent, particularly at its vertebral end : if this is borne in mind the common 

 mistake of placing the bone on the wrong side of the body will not be made. Its lower 

 margin is sharper than the upper border and has the costo-lumbar ligamentous bands 

 inserted into it : Quadratus lumborum is in front of this plane, so its insertion into the 

 bone is along the lower part of its front or inner surface, where it has a marking on the 

 bone. The marking is due to the presence of aponeurotic fibres in the muscle where it 

 lies behind the Diaphragm, and to the anterior lamella ot aponeurotic fascia covering 

 it in front. The Internal Intercostal also encroaches somewhat on this surface near the 

 top border, but with these exceptions the surface is pleural (see Fig. 35). 



But this simple arrangement of the structures about the last rib does not always hold, and 

 bones are frequently met with which show by their markings that additional relations were present 

 with them. Such a bone is shown in Fig. 45, and a sketch of a dissection is given above it which 

 represents a condition that was also probably present in the body from which the bone was taken. 



In the dissection the Quadratus lumborum is seen to give a stout aponeurotic sheet (X) (in 

 front of the last dorsal nerve (N) ), which passes up in front of the bone to reach the interrupted line 

 above A in the middle figure. This sheet is covered in the dissection by thin aponeurotic sub-costal 

 fibres running inwards and downwards to the side of the vertebral column, so that the inner end 

 of the bone is covered by these two layers of aponeurosis in front. Thus we can recognise, on the 

 front of the bone, the muscular areas for Intercostal and Quadratus lumborum muscles, and an 

 area A covered by aponeurosis passing to the ridge that bounds it above, with a pleural area B. 



On the back of the rib (lowest figure) the rough area D is for the scattered costo-transverse 

 fibres, and the lower, E, for the costo-lumbar ligaments connecting it with the first lumbar trans- 

 verse process. Outside these is a marking above for Levator costae, and, below this, irregular 

 markings showing the insertion of mixed fibres of the Erector spinx mass. This area is bounded 

 externally by secondary markings made by fibres of the definite Ilio-costalis and covering aponeu- 

 rosis. Outside this is the area C, covered by Latissimus dorsi and giving origin to some of its fibres 

 as well as to External Oblique and Serratus posticus inferior. 



The middle or main lamella of the aponeurotic origin of Transversalis is fastened t 

 border of the rib and joins the deep aspect of the costo-lumbar ligaments. 



COSTAL CARTILAGES. The costal arch or sub-costal angle is made by the 

 lower six cartilages on each side. Occasionally the eighth, however, reaches the 



