CHA I'TER VI. 



RESPIRATION. 



The means by which the blood becomes purified by passing 

 through the lungs must now engage our attention, but 

 we must first glance at the mechanism of respiration. 



The lungs occupy the whole cavity of the thorax : during 

 life no space exists between the pulmonary and costal 

 pleura, so that the case is an air-tight one. So long as this 

 air-tight condition is maintained, any movement which tends 

 to increase the size of the case, such as the retreat of the 

 diaphragm and the advance of the ribs, causes a distension 

 of the sacs and the air rushes in ; by a reversed process it 

 is pressed out, viz., by a collapse of the chest wall. If, 

 however, the cavity of the chest be opened to the external 

 atmosphere the lungs collapse, as the pressure within and 

 without is the same ; such a condition would lead in the 

 horse to asphyxia, as the pleural cavities communicate : but 

 in those animals where the right and left pleural sacs are 

 distinct, the lung on the wounded side only would 

 collapse. 



The process by which the chest is rilled with air, 

 known as Inspiration, is a purely muscular act. The 

 diaphragm, as the chief muscle of inspiration, recedes ; the 

 ribs are drawn forwards and outwards, their posterior edges 

 everted, the intercostal space widened ; by this means the 

 capacity of the chest is increased, and air rushes into the 

 lungs, distending them to the now increased capacity of the 

 chest. 



