THE MECHANICS OF THE RESPIRATORY MOVEMENTS 



463 



As the diaphragm contracts, its tendinous portion is pulled down- 

 ward, so that this septum as a whole assumes a much flatter outline 



Fig. 238. — Diagrammatic Sectionsof the Body in A, Inspiration; and B, Expiration; 

 Tr, Trachea; St, Sternum; D, Diaphragm; Ab, Abdominal Walls. The Shading 

 Roughly Indicates the Stationary Air. {From Huxley's "Lessons in Elementary 

 Physiology,^* Macmillan Co., Publishers.) 



(Fig. 239). Its shape now resembles that of a flat cone, because 

 while its tendinous part is drawn 

 downward into the abdominal 

 cavity, its contracted muscular 

 part pursues a rather straight 

 course between its place of origin 

 and its insertion. In this way, 

 the breadth of the ''comple- 

 mentary" pleural space is much 

 increased, thereby allowing the 

 tapering inferior borders of the 

 lungs to descend into it. It 

 should also be remembered that 

 the downward movement of the 

 diaphragm is greatly restricted 

 in the region of the apex of the 

 heart, because the pericardial 

 sac is here anchored to its upper 

 surface. Under ordinary condi- 

 tions, therefore, the expansion 

 of the lungs does not depend so 

 much upon the actual descent of the diaphragm as upon the enlarge- 

 ment of the complementary space. Thus, it has been observed by 



Fig. 239. — Diagram Showing the Posi- 

 tion OF the Diaphragm and Adjoining Wall 

 of the Trunk on Inspiration and Expira- 

 tion. 



E, expiration; /, inspiration. The dia- 

 phragm moves downward and the walls of 

 the trunk outward, increasing the size of 

 the complementary space C. The slight 

 depression at H is caused by the apical por- 

 tion of the heart. 



