THE MECHANISM OF RESPIRATION IN MAN 



BY ARTHUR KEITH. 



THE following is a summary of the chief points dealt with in this 

 article : 



(1) The lung is composed of elements of varying degrees of 

 extensibility ; hence the expansion of its parts are unequal 

 during inspiration. 



(2) The infundibula or air sacs are the essential distensible 

 (inspiratory) elements of the lungs. The distensibility of any part 

 of the lung will depend on the number and size of the infundibula 

 in that part. 



(3) The bronchial musculature regulates the tension of the 

 infundibular air and may regulate the distribution of air and 

 blood throughout the lung. 



(4) The lungs do not expand equally in all directions, but 

 execute a movement in certain definite directions during in- 

 spiration. 



(5) The roots of the lungs are not fixed but undergo a res- 

 piratory movement. 



(6) The great fissure of the lung is of functional significance. 

 The upper lobe is chiefly expanded by a mechanism formed by 

 the upper ribs, the lower lobe by a compound mechanism formed 

 by the diaphragm and lower ribs. 



(7) Expiration is controlled by muscular action. 



(8) The extensibility and elasticity of the thorax are factors 

 in producing expansion or compression of the lung only when 

 the ribs pass into extreme inspiratory or expiratory positions. 



(9) TLe first pair of ribs and the manubrium sterni are parts 

 of a single mechanism which may be described as the thoracic 

 operculum. The sterno-manubrial joint is of functional im- 

 portance. 



(10) In observing and analysing the respiratory movements 

 of -the thorax it is advantageous to treat the costal cartilages 



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