MOVKMKXTS OF THE THORAX AS A WHOLE 215 



strength witli elasticity is given to the wall of the trunk at a part where the 

 cartilages are the only firm structures in its composition; while a slight gliding 

 movement is permitted hetween the costal cartilages themselves, which takes place 

 round an axis corresponding to the long axis of the cartilages. By this means, 

 the outAvard projection of the lower }nirt of the thoracic Avail is increased by 

 deep inspiration. 



MOVEMENTS OF THE THORAX AS A AVHOLE 



Before describing these movements as a Avhole, it must be premised that there 

 are some feAV modifications in the movements of certain ribs resulting from their 

 shape. Thus the first rib (and to a less extent the second also), which is flat on its 

 upper and under surfaces, revolves on a transverse axis drawn through the costo- 

 vertebral and costo-transverse joints. During inspiration and ex])iration, the anterior 

 extremities of the first pair of costal arches play up and down, the tubercles and 

 the heads of the ribs acting in a hinge-like manner, the latter having also a slight 

 screwing motion. By this movement the anterior ends of the costal arches are 

 simply raised or depressed, and the sternum pushed a little forwards: it may be 

 likened to the movement of a pump-handle, as in fig. 218, «, b. 



The movements of the other ribs, particularly in the mid-region of the thorax, 

 are more complex, for, besides the elevation of the anterior extremities, the bodies 



Fig. 218. — Di.\GKA>r of Axis ok Kib-movemext. (After Kirkes.) 



and angles of the ribs rise nearly as much as the extremities themselves. In this 

 movement the tubercles of the ribs glide upwards and backwards in inspiration, 

 and downwards and forwards in expiration; and the movement may be likened to 

 that of a bucket handle, as in fig. 218, A, B. 



During inspiration, the cavity of the thorax is increased in every direction. 

 The antero-posterior diameter is increased by the thrusting forwards of the 

 sternum, caused by the elevation of the costal cartilages and fore part of the ribs, 

 whereby they are brought to nearly the same level as the heads of the ribs. The 

 transverse diameter is increased: (i) Behind, by the elevation of the middle part 

 of the rib.^; for when at rest the mid-part of the rib is on a lower level than either 

 the costo-vertebral or chondro-sternal articulations. Owing to this obliquity the 

 transverse diameter is increased when the rib is raised, and the increase is propor- 

 tionate to the degree of oljliquity. (ii) By the eversion of the lower border of the 

 costal arch, which rolls outwards as the arch is raised, (iii ) The transverse diameter 

 is increased in fi-ont by the abduction of the anterior extremity of the rib at the 

 same time as it is elevated and thrust forwards. 



The increase in the vertical diameter of the thorax is due to the elevation of 

 the riljs, especially the upper ones, and the consequent widening of the intercostal 

 spaces; but the chief increase in this direction is due to the descent of the 

 diaphragm. 



The greatest increase both in the antero-posterior and transverse diameters takes 

 place where the ribs are longest, most oblique, and most curved at their angles, and 



