3 88 TEXT-BOOK OF PHYSIOLOGY. 



dition are changed. To these movements the term respiratory has been given, 

 as a result of which the ventilation of the lungs is accomplished. 



The Respiratory Movements. The respiratory movements consist of 

 an alternate increase and decrease in the capacity of the thorax, accompanied 

 by corresponding changes in the capacity of the lungs, the two movements 

 being known as inspiration and expiration respectively. During the increase 

 in the thoracic capacity, the air passively flows into the lungs; during the 

 decrease in the thoracic capacity, the air passively flows out of the lungs. 

 In both movements the lungs play an entirely passive part, their movements 

 being determined by the pressure of air within them and by the outward 

 movement of the thoracic walls, with which they are in close contact. 



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 dificult to state at what particular instant any given muscle 

 begins to act. It is customary, however, to divide the muscles into two groups : 

 (i) Those active in the average or ordinary inspirations, and (2) those active 

 in maximum or extraordinary inspirations. Among the muscles active 

 in ordinary inspirations may be mentioned the diaphragm, the intercostales 

 externi, the inter car tilaginei, the levatores costarum, the scaleni, and the ser- 

 ratus posticus superior. Among the muscles active in extraordinary inspira- 

 tions may be mentioned, in addition to the foregoing, the sterno-cleido- 

 mastoideus, 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 in 

 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 

 (Fig. 190). The vertical diameters are thus enlarged, though unequally in 

 different regions of the thorax. 



As the diaphragm descends it displaces the abdominal viscera, forcing 



