HOW WE BREATHE 149 



again. This is the act of respiration (Lat. re, again ; and spiro, 

 I breathe). 



Fit up an apparatus as represented in fig. 135, in which A is a stout glass 

 tube, provided with a sound cork, B, and also an airtight piston, C, resembling 

 that of an ordinary syringe. A short tube, D, passing through the cork, has a 

 small india-rubber bag tied to it. Fit the cork in the tube while the piston 

 is near the top. Now, by lowering the piston we increase 

 the capacity of the cavity containing the bag. The pres- D 



sure outside the bag is thus lowered, and air rushes into 

 it through the tube D till a balance is restored. The bag is 

 thus stretched. As soon as we let go the piston, the elas- 

 ticity of the bag, being free to act, drives out the air just 

 taken in, and the piston returns to its former place. It 

 will be noticed that in this experiment the elastic bag 

 and its tube represent the lungs and trachea ; and the vessel 

 enclosing it, the thorax. If while the bag is inflated a hole 

 be made through the cork into the tube, air rushes in, 

 and the bag collapses, thus illustrating the effect of piercing 

 a hole in the walls of the chest. 



We have now to study the mechanism by 

 which the chest is made to contract and expand. 



Inspiration is a muscular act by which the 

 cavity of the chest is ividened and deepened. The 

 latter result is brought about by the contraction 

 of the diaphragm, which causes it to become less 

 arched. This contraction would also make the 

 chest narrower by exerting a pulling force on its 

 side walls to which the diaphragm is attached, but 

 this is prevented by the action of the intercostal 

 muscles. 



The intercostal muscles connect the adjacent 

 ribs throughout their length. They consist of two 

 thin layers of oblique muscular fibres which occupy 

 the spaces between the ribs. These layers are Y\g. 135. Appa- 

 termed respectively the internal and external, ratus for il- 

 The internal intercostal muscles pass obliquely lustrating the 

 downwards and backwards from the lower margin 

 of one rib to the upper margin of the next. 

 The external intercostal muscles incline downwards and for- 

 wards, and are much stronger than the internal. The ribs also 

 are inclined considerably downwards and forwards ; and as the 

 intercostal fibres are placed obliquely with regard to the ribs, when 

 they contract they tend to arrange themselves at right angles with 

 these bones, and in doing so the ribs are either raised or de- 

 pressed. The external intercostal muscles raise the ribs, 

 widening the cavity of the chest, and thus assisting inspiration. 



Movements of 



Respiration. 



