EESPIEATION 299 



lying for some distance along the inner surface of the ribs and 

 then curving inwards to be inserted into the flattened central 

 tendon, to which is attached the pericardium and on which 

 rests the heart. 



In inspiration the muscular fibres contract. But the central 

 tendon being fixed by the pericardium does not undergo ex- 

 tensive movement. The result of the muscular contraction 

 is thus to flatten out the more marginal part of the muscle 

 and to withdraw it more or less from the chest wall thus 

 opening up a space, the eomplemental pleura, into which the 

 lungs expand (fig. 136). 



It might be expected that this contraction of the diaphragm 

 would pull inwards the chest wall but this is prevented 

 by the expansion of the thorax in the lateral and vertical 

 diameters as a result of the mechanism which has next to be 

 considered. 



2nd. The Increase in the Chest in the vertical and lateral 

 diameters. 



This is brought about by the pulling forward of the ribs 

 which rotate round the axis of their attachments to the verte- 

 bral column. 



To understand this, the mode of the connection of the ribs to 

 the vertebral column must be borne in mind. The head of 

 the rib is attached to the bodies of two adjacent vertebrae. 

 The tubercle of the rib is attached to the transverse process 

 of the hinder of these vertebrae. From this the shaft of the 

 rib projects outwards, downwards and backwards, to be attached 

 below to the sternum by the costal cartilage. If the rib is 

 made to rotate round its two points of attachment, its lateral 

 margin is tilted forwards and outwards, while its lower end 

 is carried forwards and downwards (see fig. 137). 



Further, as we pass from before backwards, each pair of 

 ribs forms the arc of a larger and larger circle, and as each 

 pair rises it takes the place of a smaller pair above. In these 

 ways, the chest is increased from above downwards and from 

 side to side. 



The first pair of ribs does not undergo this movement; 

 the motion of the second pair of ribs is slight, but the 

 range of movement becomes greater and greater as we pass 

 downwards until the floating ribs are reached, and these are 



