n 



k IM 'I RATION 



119 



as we have seen, to the vertebral column. These joints allow the 

 ribs to he raised from their natural oblique position to a more 

 lioii/ontal position, that is, an up ami clown movement of the 

 front ends of the ribs can take place. When the lower ribs 

 with their larger arches arc thus raised they come into the 

 position previously occupied by the smaller arches of the ribs 

 above them. This in- 

 creases the size of the 

 thorax. The vertebral 

 column, ribs, and sternum 

 thus form a cage, the 

 shape of which can be 

 i-hanged by the up and 

 down movement of the 

 ribs on the vertebral 

 column. As the anterior 

 ends of the ribs move 

 upwards they carry the 

 sternum upwards with 

 them ; and in moving up- 

 wards, as they pass from 

 a slanting position to a. 6L- 

 more horizontal position, 

 they increase the distance 

 of the sternum from the 

 vertebral column, so that 

 by the upward movement 

 of the ribs on the verte- 

 bral column the sternum 

 is raised upwards and 

 thrust forwards. The 

 cavity of the thorax is 

 thereby increased from 

 behind forwards. 



Between the ribs arc muscles, the intercostal muscles 

 passing from one rib to the next below it. There are two layers 

 of these muscles l>etween each rib, an external layer and a 

 deeper internal layer. The ribs are raised by the contraction of 

 the external intercostal muscles. The fibres of the 

 external intercostal muscles pass from one rib obliquely down- 



FlG. 55. The bony walls of the thorax. 

 .t, /, vertebral column ; 1-12, ribs ; c, sternum ; ./, 

 costal cartilages ; e, united cartilages of lower 

 true ribs. 



