RESPIRATION IN MAN 187 



walls of the infundibula. The bronchial musculature by diminish- 

 ing or increasing the access to the infundibula in various parts 

 of the lung may regulate the distribution of the indrawn air 

 throughout the lung. By regulating the intra-alveolar pressure it 

 may also influence the distribution of blood throughout the lung, 

 and take the part of the vaso-motor mechanism which has not 

 been proved to exist in the lung. 



THE NECESSITY OF RECOGNISING SURFACES OF DIRECT EXPANSION 

 AND SURFACES OF INDIRECT EXPANSION ON THE LUNG 



The lung is usually regarded not only as being equally dis- 

 tensible in all its parts, but also of expanding equally in all 

 directions during inspiration. This is far from being true of the 

 mammalian lung. Of the five areas which one may distinguish 

 on the surface of the human lung, three are in contact with 

 stationary parts of the thoracic wall, and therefore cannot be 

 directly expanded. These three pulmonary surfaces are (1) The 

 mediastinal, in contact with the pericardium and structures of the 

 mediastinum ; (2) the dorsal surface, in contact with the spinal 

 column and with the spinal segments of the ribs those parts of 

 the ribs to which the erector spinse is attached ; (3) the apical 

 surface, the pulmonary area lying in contact with Sibson's fascia 

 at the root of the neck. It is not strictly true to say that these 

 three parts of the pleural wall are stationary, for the heart being 

 conical in shape, with its base resting on the diaphragm, it is clear 

 that the mediastinal surface of the lungs may expand inwards 

 with the inspiratory descent of the heart ; only the dorsal part 

 of the apical surface is stationary, for the ventral or anterior part 

 is elevated with the first rib, but the fact remains that during 

 inspiration the apical resonance does not extend into the neck 

 but decreases (Colbeck), and by placing a tambour on the neck 

 over the apex of the lung it is found that the apex descends when- 

 ever the diaphragm is well in action, even if the first rib remains 

 stationary. The two surfaces of the lung which are directly ex- 

 panded are the diaphragmatic and ventro-lateral or sterno-costal. 

 Thus if the diaphragm and lower ribs are in action the whole lung 

 is expanded in a downward and forward direction, the apical 

 surface remaining stationary or descending until the structures 

 of the neck become sufficiently tense to resist a further descent. 



