572 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The backbone forms a part of the posterior boundary of the chest, 

 the sternum the anterior boundary. Below, the thorax is shut off from 

 the abdominal cavity in mammals by the diaphragm ; above, it is closed 

 by the muscles of the neck. The lungs are suspended in a semi-distended 

 state in the cavity of the thorax, and with the heart and great blood- 

 vessels completely fill it. Since the thorax forms an air-tight cavity 

 there is through the elasticity of the lungs a constant tendency to the 

 production of a vacuum between the pleural surfaces. This tendency to 

 contraction, due to the elastic fibres of the lungs, is of great importance 

 in assisting respiration. If an opening be made into the pleural cavity 

 the atmosphere at once enters, and the lungs then through their elasticity 

 collapse. Atmospheric pressure being the same within and without the 

 lungs, the resistance of the walls of the thorax is the only factor which 

 prevents the collapse of the thorax from the elasticity of the lungs. 



This tendency to retraction of the thoracic walls is readily seen in 

 the intercostal spaces, particularly if the outer muscular layers be 

 removed. Distinct depressions may then be recognized between each 

 rib, and indicate the negative pressure produced upon the walls of the 

 thorax by the constant" tendency to contraction of the lungs. This 

 negative pressure is likewise exerted on the upper and lower extremities 

 of the thorax. There is, therefore, a constant depression of the soft 

 tissues of the neck toward the thoracic cavity, until the increased 

 tension so produced results in equilibrium. So, also, there is a constant 

 tendency to the ascent of the diaphragm in the thoracic cavity by the 

 same means. In the passive state of the thorax there is, therefore, a 

 constant equilibrium produced, which results from the balance between 

 this negative pressure exerted by the lungs and the resistance of the 

 walls of the thorax. 



The walls of the thorax, in so far as they are constituted by the ribs 

 and diaphragm, are not, however, rigid, but are capable of undergoing 

 change in position. 



The ribs are acted on by various muscles whose contraction results 

 in an increase in the lateral and antero-posterior diameters of the thorax. 

 The diaphragm is also capable of changing its position, but when it 

 contracts tends to depart from its concave, dome-like position, and 

 become more horizontal. The contraction of the diaphragm, therefore, 

 serves to increase the vertical diameter of the thorax. 



As the thorax is increased in its dimensions the lungs are compelled 

 to follow its movements, from the fact that otherwise there would be a 

 production of a vacuum in the pleural cavit}'. As, however, the lungs 

 likewise become increased in volume, the air in them becomes rarefied, 

 and since the air within the lungs is in direct communication with the 

 atmosphere the air streams in from without to the interior of the lungs, 



