CHAPTER V 

 RESPIRATORY MOVEMENTS 



THE object of the respiratory movements is, by alternately 

 increasing and decreasing the thoracic cavity, to suck the air 

 into the alveoli of the lungs, and after gas-exchange with 

 the pulmonary blood, to force it out again. 



1. THE CHANGE IN THE FORM OF THE THORACIC 

 CAVITY AND OF THE LUNGS 



The respiratory movements consist of the alternate increase 

 (inspiration) and decrease (expiration) of the thoracic cavity 

 in all directions. 



1 . The dilation of the thoracic cavity in the perpendicular ^ 

 diameter is produced by the contraction of the diaphragm, 

 which descends by the flattening of its convexity. In this 

 process the muscular portions play the most important part ; 

 the central tendon is of secondary importance. The per- 

 ipheral parts of the diaphragm, which, in expiration, lie 

 against the thoracic walls, are during inspiration drawn 

 away from the walls. In expiration the intestine forces the 

 diaphragm upward into the thoracic cavity. 



2. The dilation of the thoracic cavity in the horizontal 

 diameter is brought about by the elevation of the ribs. 



Each rib is movably joined to the spinal column at two places: 



1. By its head to two vertebrae. 



2. By its tubercle to the transverse process of one vertebra. 

 The axis about which the rib turns passes through its neck, hence 

 passes in almost a horizontal anterior-posterior direction. 



The ribs incline forward and downward. By their elevating the 

 degree of this inclination is lessened. By this means the cross- 



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