KESPIRATION. 179 



of a space, however, between the pleura which covers the lung (visceral 

 layer), and that which lines the inner surf ace of the chest (jwrttfoi layer), 

 is inserted in the drawing only for the sake of distinctness. These layers 

 are, in health, everywhere in contact, one with the other; and between 

 them is only just so much fluid as will ensure the lungs gliding easily, in 

 their expansion and contraction, on the inner surface of the parietal 

 layer, which lines the chest-wall. While considering the subject of 

 normal respiration, we may discard altogether the notion of the existence 

 of any space or cavity between the lungs and the wall of the chest. 



If, however, an opening be made so as to permit air or fluid to enter 

 the pleural sac, the lung, in virtue of its elasticity, recoils, and a consid- 

 erable space is left between the lung and the chest- wall. In other words, 

 the natural elasticity of the lungs would cause them at all times to con- 

 tract away from the ribs, were it not that the contraction is resisted by 

 atmospheric pressure which bears only on the inner surface of the air- 

 tubes and air-cells. On the admission of air into the pleural sac, atmos- 

 pheric pressure bears alike on the inner and outer surfaces of the lung, 

 and their elastic recoil is thus no longer prevented. 



Structure of the Pleura and Lung. The pulmonary pleura consists 

 of an outer or denser layer and an inner looser tissue. The former or 

 pleura proper consists of dense fibrous tissue with elastic fibres, covered 

 by endothelium, the cells of which are large, flat, hyaline, and transpar- 

 ent when the lung is expanded, but become smaller, thicker, and gran- 

 ular when the lung collapses. In the pleura is a lymph-canalicular 

 system; and connective tissue corpuscles are found in the fibres and tissue 

 which forms its groundworr. The inner, looser, or subpleural tissue 

 contains lamellae of fibrous connective tissue and connective tissue cor- 

 puscles between them. Numerous lymphatics are to be met with, which 

 form a dense plexus of vessels, many of which contain valves. They are 

 simple endothelial tubes, and take origin in the lymph-canalicular system 

 of the pleura proper. Scattered bundles of unstriped muscular fibre 

 occur in the pulmonary pleura. They are especially strongly developed 

 on those parts (anterior and internal surfaces of lungs) which move most 

 freely in respiration: their function is doubtless to aid in expiration. The 

 structure of the parietal portion of the pleura is very similar to that of 

 the visceral layer. 



Each lung is partially subdivided into separate portions called lobes; 

 the right lung into three lobes, and the left into two. Each of these 

 lobes, again, is composed of a large number of minute parts, called lobules. 

 Each pulmonary lobule may be considered a lung in miniature, consist- 

 ing, ^as it does, of a branch of the bronchial tube, of air-cells, blood 

 vessels, nerves, and lymphatics, with a sparing amount of areolar 

 tissue. 



On entering a lobule, the small bronchial tube, the structure of which 



