RESPIRATION 171 



the contraction of the chest, is due to the elasticity of the 

 lungs. This tendency on the part of the lungs to contract is 

 sufficient in quiet respiration to restore the chest to its usual 

 size after inspiration, and thus to expel air. The lungs are 

 held open owing to the negative pressure in the space which 

 separates them from the chest- wall. This negative pressure 

 has a most important relation to their permeability by air. 

 Imagine the condition reversed. Picture a lung into which 

 air is forced by a muscular pump. After each stroke of the 

 pump the lung would collapse. Its finest tubes and their 

 dilated terminations could be maintained as open spaces, 

 between the strokes of the pump, only by giving a consider- 

 able thickness and firmness to their walls. Such a substantial 

 structure would be unfavourable to an interchange of gases 

 between the blood and the air. The reverse of this condition 

 is found in Nature. The lung is stretched from without. Its 

 tissue, delicate as crepe, cannot collapse even at the end of 

 the deepest expiration. 



The ribs are united by intercostal muscles, disposed in two 

 sheets. The fibres of the external intercostals are directed 

 downwards and forwards, those of the internal intercostals 

 downwards and backwards. In tranquil respiration the chest 

 is enlarged by the external intercostal muscles, which raise the 

 ribs, and the diaphragmatic muscle, which renders peripheral 

 portions of the diaphragm flat. The role of the internal inter- 

 costal muscles is a subject still under discussion. For the 

 most part, physiologists regard them as accessory to expira- 

 tion, but some hold that they combine with the external inter- 

 costals in raising the ribs and twisting them outwards during 

 inspiration. The diaphragm is a partition which separates the 

 thoracic from the abdominal cavity. It is in the form of a 

 vault. The central portion of the dome is membranous, its 

 margins muscular. Its membranous centre is in contact with 

 the pericardium, which encloses the heart. The level of this 

 part is therefore fixed, except in forced inspiration, when it 

 descends slightly. It constitutes a fixed plane for the muscles 

 of the diaphragm, which are attached below to the vertebral 

 column and the ribs. When the muscles contract in inspira- 

 tion, the curvature of the marginal portions of the diaphragm 

 is diminished, and the chest-cavity consequently enlarged . 



