RESPIRATION 



401 



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



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 inspiration the thorax is enlarged in all its diameters: viz., vertical, 

 transverse, and antero-posterior. In expiration the thorax is diminished in 

 all its diameters as it returns to its former condition. 



Inspiratory Muscles. The muscles which from their origin, direction, 

 and insertion contribute to the enlargement or expansion of the thorax are 

 quite numerous, and include those muscles which enter into the formation 

 of the thoracic walls (intrinsic muscles), as well as certain muscles which, 

 having their origin elsewhere, are attached to the thoracic walls at different 

 points (extrinsic muscles), 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 custo- 

 mary, however, to divide the mus- 

 cles into two groups: (i) Those ac- 

 tive in the average or ordinary in- 

 spirations, and (2) those active in 

 maximum or extraordinary inspira- 

 tions. Among the muscles active 

 in ordinary inspirations may be 

 mentioned the diaphragm, the inter- 

 cos tales externi, the inter car tilaginei, 

 the levatores costarum, the scaleni, 

 and the serratus posticus superior. 

 Among the muscles active in extra- 

 ordinary inspirations may be men- 

 tioned, in addition to the foregoing, the sterno-cleidomastoideus, the trapezius, 

 and the pectorales minor and major. 



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, shorten, and approximate a straight line, whereby 

 not only is the convexity of the diaphragm diminished, but that portion hi 

 contact with the thorax is drawn away, thus making a large free space 

 triangular in shape, termed the complementary pleural space, into which 

 the lateral and posterior portions of the lungs at once descend. The attach- 

 ment of the central tendon of the diaphragm to the pericardium prevents 

 any marked descent of this portion except in forcible inspiratory efforts. 

 The vertical diameters are thus enlarged, though unequally in different re- 

 gions of the thorax. 



As the diaphragm descends it displaces the abdominal viscera, forcing 



them downward against the abdominal walls, which advance and become 



more convex. In forcible inspiration the diaphragm, acting from the central 



tendon as the more fixed point, would draw the lower portion of the thorax 



26 



FIG. 1 86. DIAGRAM SHOWING THE POSITION 

 AND SHAPE OF THE DIAPHRAGM AT REST a AND 

 DURING INSPIRATION a' AND b. (Boruttau.) 



