CHAPTER III 

 THORAX 



THE thoracic skeleton is attached dorsally to the vertebral column. It contains 

 a series of twelve ribs on each side, connected with a central sternum in front by means 

 of costal cartilages. 



Of each series of twelve ribs only the upper seven have their cartilages directly 

 connected with the sternum : the next three cartilages each join the one immediately 

 above, so only reach the sternum indirectly, and the two last ribs have their short 

 cartilaginous ends terminating without such junction, lying in the muscles of the 

 body wall 



The upper seven pairs are termed true or sternal ribs : the remainder are false 

 or asternal, the last two ribs on each side being sometimes referred to as " floating 

 ribs." There are sometimes eight sternal ribs. 



The wall of the thorax is completed by intercostal muscles and membranes in 

 the intercostal spaces between the ribs, and pleura lines it on its inner side. 



The ribs and sternum constitute a firm but movable thoracic cage protecting the 

 viscera. The firmness due to the bony elements obviates collapse from atmospheric 

 pressure, the lungs being expanded. Hence the intrathoracic pressure can be below 

 that of the atmosphere. The power of movement of the walls is associated, of course, 

 with respiration. 



RIBS AND COSTAL CARTILAGES. 



A bone from the middle of the series will show the general characters and various 

 parts of a rib. Such a specimen (Fig. 37) exhibits at its vertebral end an expanded 

 head (A), joined to the remainder of the bone by a somewhat narrower neck which has 

 a rough ridge, the crista colli superior (C), for the upper costo-transverse ligament. 

 The head has two articular facets, which are covered with nbro-cartilage, for the bodies 

 of the two vertebrae that carry it, separated by an inter articular crest (B) for the attach- 

 ment of the ligament that fastens it to the intervertebral disc. The neck has a smooth 

 pleural surface in front, but is ridged behind by the middle costo-transverse ligament : 

 there is often an inferior crest on the neck. 



The rest of the bone, outside the neck, is the shaft or body (D), and where this joins 

 the neck is the tubercle on the back and lower aspect of the bone : the tubercle presents 

 an inner articular part (art.), with a facet for the transverse process of its vertebra, 

 and an outer non-articular part (n. a.) for the external or posterior costo-transverse 

 ligament. 



The shaft passes outwards from the tubercle for a little distance, and then turns 

 rather sharply forwards and outwards : this turn makes the angle of the rib. Observe 

 that the angle is properly a sharp curve in the bone, but carries on its surface 

 a secondary marking made by the attachment of the Ilio-costalis and vertebral 

 aponeurosis. 



Beyond the angle the curving shaft is continued in a forward, inward and downward 

 direction : it has an upper border, thick and rounded behind and sharper in front, a 

 lower border sharp behind and rather rounded in front, an outer and an inner surface. 



