THE RESPIRATION 281 



of the abdomen. It will be described when considering the 

 mechanism by which it is brought about. 



The expansion of the chest in inspiration is a muscular 

 act and is carried out against the following forces 



1st. The elasticityjif the Lungs. To expand the lungs 

 their elastic force has to be overcome, and the more they are 

 expanded the ^eater^^s^their elasticity. This factor there- 

 fore plays a smaller part at the~Beginmng than towards the 

 end of inspiration. 



2nd. The elasticity of the Chest Wall The resting posi- 



FIG. 132. Vertical-tangential, Transverse, and Vertical Mesial Sections of the 

 Thorax in Inspiration and Expiration. 



tion of the chest is that of expiration. To expand the chest 

 the costal cartilages have to be twisted. 



3rd. The elasticity of the Abdominal Wall. As the cavity 

 of the thorax increases downwards, the abdominal viscera are 

 pushed against the muscular abdominal wall, which in virtue 

 of its elasticity resists the stretching force. 



In studying how these changes are brought about we may 

 consider 



1st. Increase in the thorax frnnm. Yy-foflf^n/y/yp^yy/ivfo. 



This is due to the contraction ollhe diaphragm (Fig. 132). 



In expiration this dome-like muscle, rising from the 

 vertebral column and from the lower costal margin,, arches 

 upwards, lying for some distance along the inner surface of 



