298 



VETERINARY PHYSIOLOGY 



abdomen. It will be described when considering the mechan- 

 ism 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 Elasticity of the Lungs. To expand the lungs 

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

 expanded the greater is their elasticity. This factor therefore 

 plays a smaller part at the beginning than towards the end of 

 inspiration. 



2nd. The Elasticity of the Chest Wall. The resting position 



FIG. 136. Vertical-tangential, Transverse, and Vertical Mesial Sections of 

 the Thorax in Inspiration and Expiration. 



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 backwards, 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 



ls. The Increase in the Thorax from before backwards. 



This is due to the contraction of the diaphragm (fig. 136). 



In expiration this dome-like muscle, rising from the vertebral 

 column and from the posterior costal margin, arches forwards, 



