148 ELEMENTARY PHYSIOLOGY 



is firmly attached to the outer surface of the organs, while the 

 other lines the cavity of the chest, being closely adherent to its 

 walls. 



Thus the chest is an air-tight cavity, having no direct communi- 

 cation with the atmosphere ; while the lungs may be considered as 

 bags filling this cavity, being themselves filled out with air which 

 enters through the trachea. 



Although in fig. 78 a space is represented between the two 

 layers of the pleura, this is not actually the case, the space 

 being inserted only for the sake of distinctness : the two layers 



of the pleura are practically 

 in contact with each other, 

 there being nothing between 

 them save a very little watery 

 fluid (serum) which is se- 

 creted by the membrane, 

 and only sufficient of this to 

 allow of a free gliding motion 

 during the respiratory move- 

 ments of the chest and lungs. 

 Such a secreting closed sac 

 is called a serous membrane. 

 If a hole be made in the 

 walls of the chest, the lungs 

 immediately collapse, be- 

 coming considerably smaller; 

 and while the chest is thus 

 perforated breathing is very 

 difficult, or, if the opening is 



Fig. 134. The Framework of the Chest, large, quite impossible. It 



is the natural elasticity of the 

 lungs which causes them to 

 shrink away from the walls 

 of the chest ; hence it must be that in their normal condition 

 they are stretched by some force which is able to overcome 

 this elasticity. 



Respiration is the result of the alternate expansion and con- 

 traction of the walls of the chest. In inspiration (Lat. in, and 

 spiro, I breathe) we enlarge the chest by means of certain muscles, 

 and so tend to produce a vacuum (an empty space) between the 

 lungs and the chest ; and, as a natural result, air rushes in through 

 the trachea, thus causing the lungs to expand with the chest. 

 When these muscles cease to contract, the elasticity of the lungs 

 causes those organs to return to their former volume. The walls 

 of the chest follow the lungs, and the air taken in is expelled 



, first dorsal vertebra ; B, last (i2th) dorsal verte- 

 bra ; i to 12, ribs ; s, sternum ; c, costal carti- 

 lages. 



