EESPIEATION 295 



of the various parts of the respiratory tract must be studied 

 practically.} 



Each sac is lined by a layer of simple squamous epithelium, 

 supported by a framework of elastic fibrous tissue richly 

 supplied with blood vessels. It has been calculated that, if 

 all the air vesicles in the lungs of a man were spread out in 

 one continuous sheet, a surface of about 100 square metres 

 would be produced and that the blood capillaries would occupy 

 about 75 square metres of this. 

 Through these vessels about 

 5000 litres of blood would 

 pass in twenty-four hours. 



The larger air passages are 

 supported by pieces of hyaline 

 cartilage in their walls, but the 

 smaller terminal passages, the 



bronchioles, are without this FIG. 134. Scheme of the Distribu- 



support, and are surrounded tiou of a Bronchiole, Infundibular 



. I-, j , , Passage, and Air Sacs of the Lung 



by a specially well-developed 



circular band of non-striped muscle the bronchial muscle 



which governs the admission of air to the infundibula and air 



sacs. 



The lungs are packed in the thorax round the heart, com- 

 pletely filling the cavity. 



They may be regarded as two compound elastic-walled sacs, 

 which completely fill an air-tight box with movable walls 

 the thorax and communicate with the exterior by the wind- 

 pipe or trachea. 



No air exists between the lungs and the sides and base of 

 the thorax, so that the so-called pleural cavity is simply a 

 potential space. If the thoracic wall be punctured so that 

 this potential pleural cavity is brought into connection with 

 the air, the lungs immediately collapse and occupy a small 

 space posteriorly round the large bronchi. This is due to 

 their elasticity (fig. 135). 



The lungs are kept in the distended condition in the thoracic 

 cavity by the atmospheric pressure within them. 



Their elasticity varies according to whether the organs are 

 stretched or not. As they collapse, their elastic force naturally 

 becomes less and less; as they are expanded, greater and 



