174 HANDBOOK OF PHYSIOLOGY. 



form a closed sac, as in the case of other serous membranes, will be best 

 understood by reference to Fig. 148. The appearance of a space, how- 

 ever, between the pleura which covers the lung (visceral layer), and that 

 which lines the inner surface of the chest (parietal 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 insure 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 con- 

 siderable space is left between it and the chest-wall. In other words, 

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



FKJ. 149. Ciliary epithelium of the human trachea, o, Layer of longitudinally arranged elastic 

 fibres; 6, basement membrane; r, deepest cells, circular inform; d, intermediate elongated cells; 

 e, outermost layer of cells fully developed and bearing cilia. X 350. (Kolliker.) 



tract away from the ribs, were it not that t-he 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, at- 

 mospheric 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 transparent 

 when the lung is expanded, but become smaller, thicker, and granular 

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

 and connective-tissue corpuscles are found in the fibrous tissue which 

 forms its groundwork. The inner, looser, or subpleural tissue contains 

 lamellae of fibrous connective tissue and connective-tissue corpuscles be- 

 tween them. Numerous lymphatics are to be met with, which form a 

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



