636 



Journal of Agriculture. 



[10 Oct., 1908. 



affected side will, by virtue of its elasticity, collapse, and air will collect 

 between the pleural lining of the lung and that of the chest wall. The 

 lungs it must be remembered contain no muscle so that they are incapable 

 of spontaneous movement. Some of the lower air-breathers such as the 

 frog inflate the lung by forcing air in through a muscular effort in the 

 mouth, akin to swallowing. In all the higher vertebrates however, air is 

 admitted by the device of enclosing the lungs in an air-tight box — the 

 thorax — v/hich latter can be altered in capacitv through muscular effort 



Fig. 54. Horizontal Section of the Horse's Chest, looked at from above, illustrating 

 the Movements of the Diaphragm. (After Su^indorf.) 



rt, right lung-; b, left lung. 1. Position of the diaphragm during deep expiration; c, liver 

 d, stomach ; «?, spleen. 2, Position of diaphragm during dee]) inspiration ; c', liver : </', stomach 

 ■e', spleen ; /, po.sterior vena cava as it passes through the diaphragm. 



When the thorax expands the surface of the lung follows the retreating 

 thorax wall and so air is sucked in through the only inlet possible, namely, 

 the trachea. The thorax can be expanded by two methods. Between the 

 thorax and the abdomen is a dome-shaped sheet of muscle called the 

 midriff or diaphragm. (Fig. 54). Within the dome are portions of the 

 abdominal viscera, whilst in contact with the convex surface is the heart 

 with a lung on either side. When the muscle, of which the midriff is 

 largely composed, contracts, the dome is flattened particularly at the sides, 

 and thus the thorax increases at the expense of the abdomen. The second 

 method of expanding the thorax is by alteiing the position of the ribs. 

 This is a complex movement not easily described and best understood by 

 watching one of the larger animals, especially in laboured breathing. The 

 ribs will be seen to move headwards and outwards, increasing the chest 

 girth to a very appreciable extent. The reverse act, namely expiration, 

 when it takes place in quiet breathing, does not demand any muscular effort ; 

 all that is required is that the muscular activities producing inspiration 

 should cea.se. The abdominal contents l>eing under pressure force the 

 diaphragm towards the thorax. Further, the weight and the elasticity of 

 the thorax and the elasticity of lung combine towards the same end, 

 namely, the expulsion of some of the air-content of the lungs. But when 



