216 



ACTION OF THE INDIVIDUAL RESPIRATORY MUSCLES. 



3. Muscles Acting upon the Sternum, the Clavicle, and the Spinal Col- 

 umn. If the head be held in a fixed position by the muscles of the back of 

 the neck, the sterno-cleido-mastoid can enlarge the thorax in an upward 

 direction by raising the manubrium, together with the sternal extremity 

 of the clavicle, thus assisting the scalene muscles. In like manner, but 

 to a lesser extent, the clavicular insertion of the trapezius .may become 

 efficient. A stretching of the dorsal portion of the vertebral column must 

 result in an elevation of the upper ribs and a widening of the intercostal 

 spaces, by means of which the inspiratory capacity is substantially in- 

 creased. During deep inspiration this stretching is effected involuntarily. 



4. In forced respiration every inspiration is accompanied by a 

 descent of the larynx and a widening of the glottis. At the same time 



the palate is raised, in order 

 to allow the air to enter 

 with the least possible resist- 

 ance. 



5. Forced respiration is 

 first made evident in the 

 face by an inspiratory dila- 

 tation of the nostrils (horse, 

 rabbit). During marked 

 dyspnea the cavity of the 

 mouth is enlarged with each 

 inspiration by a dropping of 

 the jaw (gasping). 



B. Expiration. Quiet 

 expiration is accomplished 

 without muscular effort. It 

 is, first of all, dependent 

 principally upon the weight 

 of the thorax, which has a 

 tendency to fall back from 

 its elevated position to the 

 lower expiratory position. 

 This is assisted by the elas- 

 ticity of the various parts. 

 When the costal cartilages 

 are elevated, their lower 

 borders are slightly rotated 

 from below forward and up- 

 ward, and their elasticity is thus brought into play. Hence, as soon as 

 the inspiratory forces are relaxed, the cartilages return to their lower 

 and no longer distorted expiratory position. At the same time, the 

 elasticity of the distended lungs draws the thoracic walls, as well as 

 the diaphragm, together on all sides. Finally, the tense, elastic ab- 

 dominal walls, which become stretched and pushed forward, especially 

 in men, return to their non-distended state of rest when the pressure 

 of the diaphragm from above is released. It is self-evident that when 

 the body is in an inverted position, the effect of the weight of the thorax 

 is removed, and is replaced by the weight of the abdominal viscera 

 pressing upon the diaphragm. 



Among the muscles that are brought into action only during forced 



ii 



FIG. 81. I, II. Diagrammatic Representation of the Mechanism 

 of the Intercostal Muscles. 



