172 THE HUMAN MECHANISM 



In life the pleural cavity is enlarged during inspiration by 

 the contraction of the diaphragm and the elevation of the 

 ribs. Both of these are movements effected by the action of 

 skeletal muscles. The understanding of the elevation of the 

 ribs need give no difficulty ; muscles, some of which are 

 shown in Fig. 12, pull upwards on the ribs; and the attach- 

 ment of the ribs to the vertebral column and the breastbone 

 (sternum) is such that when they are raised the diameter of 

 the thorax is increased dorsoventrally and from side to side. 

 The diaphragm, on the other hand, is a kind of circular .mus- 

 cle with a central fibrous or tendinous portion from which 

 the bundles of muscle fibers radiate outwards to its edges. 

 Any shortening of these fibers evidently diminishes the 

 diameter of the diaphragm ; and because of its form (that 

 of a dome directed upwards into the thoracic cavity), 

 contraction of this muscle must increase the size of the 

 lower thorax. 1 



There are three typical modes of breathing : (1) The pre^ 

 dominantly costal, or " rib," breathing. Here the diaphragm 

 is but little used. It is the type characteristic of those who 

 impede movements of the lower ribs and abdomen with 

 constricting clothing, such as tight corsets. (2) The pre- 

 dominantly abdominal. Here the ribs are little used, while 

 the diaphragm does most of the work, the abdominal mus- 

 cles being relaxed so that the belly wall has its maximum 

 of movement. This type of breathing .involves great relax- 

 ation of tone of the abdominal muscles, which is a serious 



1 The action of the diaphragm is often described as increasing the antero- 

 posterior (head to foot) dimension of the thorax ; but this can happen only 

 when the diaphragm is free to descend, and it can descend only when, by 

 displacing downwards the contents of the abdominal cavity, it causes the 

 well-known respiratory movements of the abdominal walls. These "abdomi- 

 nal movements" may, however, be prevented by the simultaneous contrac- 

 tion of the abdominal muscles. In this case the diaphragm cannot descend, 

 and its contraction can only raise the lower ribs to which it is attached. 

 The mechanism in these two methods of using the diaphragm is clear 

 from Fig. 80. 



