THE MECHANICS OF RESPIRATION 



319 



The Action of the Musculature of the Ribs 



In a general way, the external intercostal muscles may be considered 

 as a broad extension of the scalene muscles over the thoracic walls, with 

 the ribs as intersections. The sealeni serve to fix the position of the 



Fig. 113. Intercostal muscles of 5th and 6th spaces. A, side view; B, back view; IV, 4th 

 dorsal vertebra; V, 5th rib and cartilage; /, // M. levatores costarum, 2, 2, external intercostals; 

 3, 3, internal intercostals, exposed by removal of the external muscles. In A, there are no external 

 intercostals in the' intercartilaginous spaces; in B there are no intercostals near the vertebral 

 column. (From Allen Thomson.) 



first rib so that it forms an anchorage for the action of the external 

 intercostal muscles in raising the lower ribs. They also raise the upper 

 three pairs of ribs along with the manubrium and sternum. 

 The function of the intercostal muscles has been the subject of much 



Fig. 114. Hamberger's schema to demonstrate the functional antagonism of internal and ex- 

 ternal intercostals. 



When the ribs ac and bd pass into the inspiratory positions ag and bf, the intercostal space 

 dilates (bh is greater than a&) ; the sternum gf moves away from the vertebral column ab (bf is 

 greater than be) ; the fibers of the external intercostals ak shorten (ak is greater than a/) ; and 

 those of the internal intercostals ck lengthen (ck is greater than Ig). The reverse occurs when 

 the inspiratory position is taken. (From L,uciani's Human Physiology.) 



