STRUCTURE OF DIAPHRAGM 303 



would expect to find that the lungs collapse. This is not always 

 so. A further force comes into play. Moistening the various 

 surfaces is the lymphatic secretion already referred to and, by the 

 force of surface tension, the lungs are held to the chest wall, just 

 as firmly as a boy's leather " sucker " is held to the pavement 

 and for the same reason. 



Mechanics of Respiration. 



During inspiration the capacity of the thorax is increased in all 

 directions. That expansion occurs laterally and in an antero- 

 posterior direction may be made manifest by measurement or by 

 moulding strips of lead (cyrtometers) to the circumference of the 

 chest. The movements in a vertical plane have been studied by 

 means of the X-rays and by percussion. If the intercostal spaces 

 are tapped with the finger, a clear resonant note will be emitted 

 when the percussion has been performed on a part overlying 

 inflated lung. Otherwise a dull sound will be produced. Hori- 

 zontal expansion is obtained by movements of the ribs while the 

 vertical movements are caused by contraction of the diaphragm. 



I. Structure of the diaphragm. This is a vaulted musculo- 

 fibrous sheet separating the thorax from the abdomen. It 

 consists of a central tendon like a double-arched cupola which is 

 attached on its thoracic surface to the pericardium and marginally 

 to the thoracic walls by muscles. These diaphragmatic muscles 

 may be divided into two sets, (i) crural and (ii) costal. The 

 former have their origin in the three or four lumbar vertebrae and 

 in the arcuate ligaments and are inserted into the posterior margin 

 of the central tendon, while the latter arise from the cartilages and 

 lower six ribs and from the back of the ensiform process. Such 

 a division of the muscle into crural and sterno-costal portions is 

 supported not only (1) by their different origins, but (2) by their 

 development from different muscular sheets in the embryo ; 

 (3) by their different blood supply the former directly from the 

 aorta and the latter from the intercostal and internal mammary 

 arteries ; and (4) by their different nerve supply, the crus being 

 served by the posterior branch of the phrenic nerve and the costal 

 by the anterior branch. Moreover, the two portions act some- 

 what differently, and further, people may be classed as having 

 respiration of a crural or of a parietal type depending on whether 

 the crural or the costal portions of the diaphragm are employed 

 during quiet breathing. The majority of individuals employ 

 both parts of the muscle in varying degrees. 



