282 HUMAN PHYSIOLOGY 



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 bein^ lix i d l>y the pericardium does not 

 undergo extensive 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 complemental pleura, 

 into which the lungs expand. 



The existence of this complemental pleura is of great 

 importance in cases of accumulation of fluid in the chest, 

 for the fluid may collect and push down the diaphragm 

 without materially altering the lower margin of the lungs. 



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 antero-posterior diameters as a result of the mechanism 

 which has next to be considered. Nevertheless, in young 

 children, in whom the chest wall is soft, an indrawing of the 

 chest with each contraction of the diaphragm may occur, 

 and may lead to permanent distortion of the thorax. 



2nd. T^wpfiAP j^L-Jh^^JiffSt JAL in^Afi g/n,tf i ,r'o- f po8t(>Tior d'nd 

 lateral diameters. 



This is brought aboqt by the elevation of the ribs which 

 rotate round the axis of their attachment > in the vertebral 

 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 lower of these vertebrae. From this the shaft of the 

 rib projects outwards, downwards and forwards, to be attached 

 in front to the sternum by the costal cartilage. If the rib 

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

 lateral margin is elevated and carried outwards, while its 

 anterior end is carried forwards and upwards (see Fig. 133). 



Further, as we pass from above downwards, 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 



