RESPIRATION. 347 



fall of intra- thoracic pressure at the end of a quiet inspiration amounts 

 to about 9 mm. Hg. In forcible inspiratory efforts this fall in intra- 

 thoracic pressure may amount to 30 or 40 mm. of Hg. As the intra- 

 pulmonary pressure rises above the atmospheric pressure during 

 expiration, the recoil of the elastic tissue is again opposed, with the 

 result of increasing the intra-thoracic pressure, though not in a 

 steadily progressive manner. The changes in intra-thoracic pressure 

 are graphically represented in the lower half of Fig. 167. 



Respiratory Movements. The ventilation of the lungs is 

 accomplished by an alternate expansion and contraction of the 

 thorax, accompanied by corresponding changes in the lungs, the two 

 movements being known as inspiration and expiration. In both 

 movements the lungs play an entirely passive part, their movements 

 being determined by the thoracic walls, with which they are in close 

 contact. 



1. Inspiration is an active process, the result of muscle activity, 



in consequence of which atmospheric air flows into the lungs. 



2. Expiration is a passive process, the result mainly of the recoil of 



the elastic tissue of the walls of the thorax and abdomen and of 

 the elastic tissue of the lungs, in consequence of which the intra- 

 pulmonary air flows out of the lungs. 



In inspiration the thorax is enlarged in all its diameters: viz., verti- 

 cal, antero-posterior, and transverse. In expiration these diameters 

 are again decreased as the thorax returns to its previous condition. 



Inspiratory Muscles. The muscles which from their origin, 

 direction, and insertion contribute to the enlargement or expansion of 

 the thorax are quite numerous, though the extent to which they are 

 called into activity depends on the necessity for either tranquil or 

 energetic inspirations. The gradations between a minimum and a 

 maximum inspiration are very slight, and it is difficult to state at 

 what particular instant any given muscle begins to act. It is custom- 

 ary, however, to divide the muscles into two groups: (i) Those active 

 in the average or ordinary inspirations, and (2) those active in maxi- 

 mum or extraordinary 'inspirations-y^Among the muscles active in 

 average inspirations may be mentioned the diaphragm, the scaleni, 

 trie" tevatoresjgstarum, the serratus posticus superior, the intercostales 

 externi, and the intercartilaginei.JjpAmoYLg the muscles active in 

 forced inspirations may be mentioned, in addition to the foregoing, 

 the sterno-cleido-mastoidei, the trapezei, and the pectorales minor es 

 and ma j ores.// 



The Diaphragm. The vertical diameter is increased by the 

 contraction and descent of the diaphragm, and more especially of its 

 lateral muscular portions. At the end of an expiration the diaphragm 

 is relaxed, and the lower portion closely applied to the walls of the 

 thorax. At the beginning of an inspiration the muscle-fibers contract, 



