392 EXTERNAL RESPIRATION 



pleura arc always uornially in close contact with one another 

 and with the underlying and overlying surfaces. In other words, 

 the chest wall, the two layers of the pleura and the outer surface 

 of the lungs move almost as one structure. The elasticity of 

 the lungs has been determined as about 30 mm. Hg. If this 

 inwards pull of the pulmonary tissue be subtracted from the 

 atmospheric pressure (760 mm.) in the lung, the resulting figure 

 (730 mm.) represents the force tending to keep the lungs expanded. 

 If, now, communication be established between the outer air and 

 the intra-pleural cavity, there will be a pressure of 760 mm. 

 tending to cause the lungs to collapse. As these outwards and 

 inwards pressures (760 as against 730 mm.) do not balance, one 

 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 w^iich 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 ; 



