212 PHYSIOLOGY FOR DENTAL STUDENTS. 



ticulated posteriorly with the vertebral column and anteriorly 

 with the sternum, forms a ring directed obliquely from behind 

 forwards and downwards. Any muscles whose action would 

 bring about a raising of the anterior ends of the ribs, would 

 therefore lessen the oblique position and increase the distance be- 

 tween each pair of ribs, and also add to the anterior posterior 

 diameter of the thorax. Moreover each rib increases in length 

 from above downwards, and as the ribs are raised, the lower 

 longer rib occupies the place previously held by its shorter neigh- 

 bor. This movement therefore causes the dome or apex of the 

 thorax to become more flat and broad. Moreover the lower ribs 

 are so articulated with the spinal column that they exhibit an up- 

 ward rotary movement, which resembles that made by a bucket 

 handle, and which increases the lateral or transverse diameter 

 of the thorax. 



The muscles which are responsible for the inspiratory eleva- 

 tion of the ribs are mainly the external intercostals, aided by 

 other muscles of the thorax, some of which are called into use 

 only when very powerful respiratory movements are necessary. 



Normal expiration is almost entirely a passive act. The re- 

 coil of the stretched elastic tissue of the lungs, after the in- 

 spiratory muscles have ceased to act, returns the diaphragm and 

 thoracic cage to the expiratory position. This is aided somewhat 

 by the actions of the internal intercostal muscles which lower the 

 ribs. By increasing the size of the thoracic cavity, inspiration 

 causes a corresponding increase in volume of the thoracic organs : 

 viz., the lungs and the vascular structures, because the thorax is 

 a closed cavity, so that when it expands it must either produce a 

 vacuum between the organs which fill it and its own walls, or the 

 volume of the organs must increase. It is the latter process 

 which mainly occurs, the result being that air is pushed into the 

 lungs by the atmospheric pressure whenever the thoracic cavity 

 is increased in size. 



THE MOVEMENTS OF THE LUNGS. The changes produced in 

 the size of the thoracic cavity and the lungs during normal res- 

 piration or in disease, are easily determined by noting the sounds 

 which are produced by tapping or percussing with the fingers the 



