204 THE MECHANISM OF 



RESPIRATORY MOVEMENTS OF THE UPPER RIBS 



The upper ribs differ from the lower set (1) in their muscu- 

 lature ; (2) in their articulation and ligaments ; (3) in their shape 

 and arrangement ; (4) in their movements. Their movement is 

 designed for the expansion of that part of the lung which lies 

 above the great fissure. The musculature of these ribs is the inter- 

 costal and interchondral. The first rib and its muscles provide a 

 fulcrum towards which the upper set of ribs may be raised, while 

 the lower set of ribs, fixed by the abdominal musculature, affords 

 a fixed base towards which they may be depressed during expira- 

 tion. Now the spaces between the ribs of the upper set are 

 peculiarly wide on the anterior and lateral aspect of the chest 

 the area covered by the pectoral muscles and the upper part of 

 the serratus magnus. It is in these spaces that most observations 

 on the action of the intercostal muscles have been made. Duchenne 

 noted that during life the musculature of these spaces was tense 

 during inspiration and lax during expiration in the living, and 

 inferred that both external and internal intercostals were in action 

 during inspiration, and out of action during expiration. That was 

 also Haller's opinion. Rutherford and many others also drew 

 that inference from experiments on the articulated thorax. Sibson, 

 who was an accurate observer, also agreed that the internal inter- 

 costals of the subpectoral region of the chest are inspiratory in 

 function. Both internal and external muscles have the power 

 of diminishing the intercostal spaces, and may therefore act as 

 inspiratory or expiratory muscles according to whether they act 

 from the first rib or from the sixth. One must receive with 

 caution the inferences drawn from experiments which involve a 

 wide disturbance of the circulatory and reflex mechanisms such 

 as those of Martin and Hartwell. They found the interchondral 

 as well as the internal intercostal muscles were expiratory, a con- 

 clusion altogether at variance with the fact that these cartilages 

 are elevated during inspiration. In other parts of the thorax, 

 with the exception of the anterior and lateral aspect of the upper 

 part of the thorax, the internal intercostals are normally expiratory 

 muscles. 



The spinal articulations of the ribs of the upper set differ from 

 those of the lower set. The articulation on the tubercle is a con- 



