514 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



when the shoulders are fixed by the trapezei and the rhomboidei. The erectores 

 spince further assist this action by extending the spinal column. 



Movements of Expiration. During quiet breathing expiration is effected 

 mainly or solely by the passive return of the displaced parts. Normal expi- 

 ration is therefore essentially a passive act, although it may be assisted by the 

 contraction of the interosseous portion of the internal intercostals. The most 

 important factors are unquestionably the elastic tension of the lungs, costal 

 cartilages, intercostal spaces, and abdominal walls, together with the weight of 

 the chest. 



The lungs after quiet expiration are in a state of elastic tension equal to a 

 pressure of -f 1.9 to -f 3.9 millimeters of mercury (see p. 505), which pressure 

 during inspiration is increased in proportion to the depth of the movement. 

 As soon, therefore, as the inspiratory muscles cease to contract, this tension 

 comes into play, and, aided by elastic and mechanical reactions below noted, 

 forces air from the lungs. This elasticity, and the facility with which the air 

 is expelled, may be demonstrated by inflating a pair of excised lungs and then 

 suddenly allowing a free egress of the air : collapse occurs with remarkable 

 rapidity, with a force proportionate to the degree of distention. The elastic 

 costal cartilages are similarly put on the stretch : the lower borders are drawn 

 outward and upward and are thus twisted out of position, so that as soon as 

 the inspiratory forces are withdrawn they must untwist themselves, further 

 aiding the elastic reaction of the lungs. The intercostal spaces, excepting the 

 first two, are widened and the tissues are stretched, and the diaphragm during 

 its descent presses upon the abdominal viscera, rendering the abdominal walls 

 tense. When, therefore, inspiration ceases the reaction of the tense and elastic 

 intercostal tissues aids in bringing the chest into the position of rest, while the 

 stretched abdominal walls press upon the abdominal viscera and thus force 

 the diaphragm upward. Finally, the chest-walls by their weight tend to fall 

 from the position to which they have been raised, adding thus another factor 

 toward the elastic reaction of the lungs, costal cartilages, intercostal tissues, and 

 abdominal walls. 



Whether or not the interosseous portion of the internal intercostal muscles 

 assists in expiration cannot be stated with positiveness. The fact that these 

 muscles contract during the expiratory phase and that the contraction results 

 in an approximation of the ribs leads to the belief that they are expiratory. 

 But, as before stated (p. 512), this activity may be primarily for the purpose 

 of maintaining a proper degree of tension of the intercostal tissues. In the 

 dog these muscles are not active until dyspnoea appears, while in the cat they 

 do not come into play until extreme dyspnoea has set in (Martin and Hartwell). 

 These facts certainly militate against regarding them as active expiratory fac- 

 tors during quiet breathing, while during forced expiration they may with 

 accuracy be considered as being in part at least expiratory in function. We are 

 therefore justified in concluding that normal quiet expiration is essentially a 

 passive act due to elastic reaction and to the mechanical replacement of dis- 

 placed parts. 



