278 HUMAN PHYSIOLOGY 



passages (infundibular passages) terminate. (The structure 

 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. 

 FIG. 130. Scheme of the Distribution The larger air passages are 



su f portedby pieces of hyaline 

 cartilage in their walls, but the 

 smaller terminal passages, the bronchioles, are without this 

 support, and are surrounded 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 windpipe 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. 131). 



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 natu- 



