THE LUNGS. 1091 



xracic aorta, the aortic intercostal arteries, the azygos, hemiazygos and accessory 

 hemiazygos veins, the thoracic duct and the oesophagus, with the two vagi. 



Structure of the Pleura. The pleura on each side is a closed sac, and, like other 

 serous membranes, is attached to the wall of the cavity which it lines and to the 

 surface of the viscus which it covers. It is composed of a thin connective- tissue stratum, 

 in which bundles of fibres cross each other in various directions, and intermixed with 

 which there is a considerable quantity of elastic tissue. On the internal surface of this 

 there is a continuous coating of thin endothelial cells placed edge to edge. The pleura 

 so formed is attached to the parts which it lines and invests by a small amount of areolar 

 tissue termed the subserous layer. In the case of the pulmonary pleura the subserous 

 tissue is continuous with the areolar tissue in the substance of the lung, and this 

 accounts for the tight manner in which the membrane is bound down. 



The pleura is plentifully supplied with blood. This is conveyed to it by. minute twigs 

 from the intercostal arteries, the internal mammary artery, and the bronchial arteries. 

 Lymph vessels are also particularly abundant in the pleura and in the subserous 

 layer, and it is by these that excess of fluid is conveyed from its cavity. Many lymph 

 vessels communicate directly with the cavity by means of excessively minute orifices 

 termed stomata. Dybkowsky has shown that the lymph vessels and stomata of the 

 pleura are not equally distributed throughout the membrane. Over the ribs and on the 

 mediastinal pleura they are absent. 



PULMONES. 



The Lungs. When healthy and sound each lung lies free within the corre- 

 sponding pleural cavity, and is attached only by its root and the ligamentum 

 pulmonale. It is uncommon, however, in the dissecting room, to meet with a 

 perfectly healthy lung. Adhesions between the pulmonary and parietal layers 

 of pleura, due to pleurisy, are generally present. 



Like the cavities in which they are placed, the two lungs are not precisely 

 alike. The right lung is slightly larger than the left, in the proportion of about 

 11 to 10. The right lung is also shorter and wider than the left lung. This 

 difference is due partly to the great bulk of the right lobe of the liver, which forces 

 the right cupola of the diaphragm to a higher level than the left cupola, and 

 partly to the heart and pericardium projecting more to the left than to the right, 

 thus diminishing the width of the left lung. 



The lung is light, soft, and spongy in texture ; when pressed between the 

 finger and thumb it crepitates, and when placed in water it floats. The elasticity 

 of the pulmonary tissue is very remarkable. A striking demonstration of this is 

 afforded when the thoracic cavity is opened, and the atmospheric pressure acting 

 upon the interior and exterior of the lung is equalised. Under these* conditions 

 the organ immediately collapses to about one-third of its original bulk, and it 

 becomes impossible in such a specimen to study its proper form and dimensions. 



The surface of the adult lung presents a mottled appearance. The ground 

 colour is a light slate -blue, but scattered over this there are numerous dark 

 patches of various sizes, and also fine dark intersecting lines. The coloration of 

 the lung differs considerably at different periods of life. In early childhood the 

 :luug is rosy-pink, and the darker colour and the mottling of the surface, which 

 i appear later, are due to the pulmonary substance, and particularly its interstitial 

 areolar tissue, becoming impregnated, more or less completely, with atmospheric 

 I iust and minute particles of soot. 



At every breath foreign matter of this kind is inhaled, but only a small proportion of it 



n eaches the lung tissue. The greater part of it becomes entangled in the mucus which 



'ioate the mucous membrane of the larger air - passages, and is gradually got rid of along 



ith the mucus through the activity of the cilia attached to the lining epithelium. By the 



;onstant upward sweep of these a current towards the pharynx is established. The fine dust and 



>oot particles which reach the finer recesses of the lungs, and ultimately the interstitial tissue, are 



>artly conveyed away by the lymph vessels to the bronchial glands, which in consequence 



oecome, in many cases, quite black. The colour of the lung, therefore, depends, to some 



extent, upon the purity of the atmosphere which is inhaled, and it thus happens that in coal- 



nmers the surface of the lung may be very nearly uniformly black. 



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