MOVEMENTS OF KESP1RATION. 



275 



Movements of Respiration. 



The air which is contained in the pulmonary lobules and vesicles, 

 being used for the purpose of arterializing the blood, becomes rapidly 

 vitiated in the process of respiration, and requires accordingly to be as 

 rapidly expelled and replaced by a fresh supply. This exchange or 

 renovation of the air is effected by alternate movements of the chest, of 

 expansion and collapse, which follow each other in regular succession, 

 and which are known as the " movement of inspiration," and the " move- 

 ment of expiration." 



Movement of Inspiration. The expansion of the chest is effected by 

 two sets of muscles, namely, the diaphragm and the intercostals. 

 While the diaphragm is relaxed, it has the form of a vaulted partition, 

 the edges of which are attached to the inferior extremity of the sternum, 

 the inferior costal cartilages, the borders of 

 the lower ribs and the bodies of the lumbar 

 vertebrae, while its convexity rises into the 

 cavity of the chest, as high as the level of 

 the fifth rib. When the fibres of the dia- 

 phragm contract, their curvature is neces- 

 sarily diminished; and they approximate a 

 straight line, in proportion to the extent of 

 their contraction. Consequently, the entire 

 convexity of the diaphragm is diminished in 

 the same proportion, and it descends to- 

 ward the abdomen, enlarging the cavity of 

 the chest from above downward. At the 

 same time the intercostal muscles enlarge it 

 in a lateral direction. For the ribs, articu- 

 lated behind with the bodies of the vertebrae, 

 and attached to the sternum by the flexible 

 and elastic costal cartilages, are so arranged 

 that, in a position of rest, their convexities 

 look obliquely outward and downward. 

 When the movement of inspiration is about 

 to commence, the first rib is fixed by the 

 contraction of the scaleni muscles, and, the 

 intercostal muscles then contracting simul- 

 taneously, the ribs are drawn upward. In 

 this movement, as each rib rotates upon its 

 articulations with the spinal column at one 

 extremity and with the sternum at the 

 other, its convexity is necessarily carried 

 outward at the same time that it is drawn 

 upward, and the parietes of the chest are 

 expanded laterally. The sternum rises slightly with the same move- 

 ment, and enlarges to some extent the antero-posterior diameter of the 



DIAGRAM ILLUSTRATING 

 THE RESPIRATORY MOVE- 

 MENTS. a. Cavity of the chest. 

 6. Diaphragm. The dark out- 

 lines show the figure of the chest 

 when, collapsed ; the dotted lines 

 show the same when expanded. 



