

RESPIRATION. 257 



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

 the inner surface of the chest ( parietal layer), is inserted in the draw- 

 ing only for the sake of distinctness. These layers are, in health, every- 

 where in contact, one with the other; and between them is only just so 

 much fluid as will insure gliding easily, in their expansion and contrac- 

 tion, 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 it and the chest-wall. In other words, the 

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



Fig. 204. Transverse section of the chest. 



away from the ribs were it not that the contraction is resisted by atmos- 

 pheric pressure which bears only on the inner surface of the air-tubes 

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

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

 their elastic recoil is thus no longer prevented. 



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 ex- 

 panded, but become smaller, thicker, and granular when the lung col- 

 lapses. In the pleura is a lymph-canalicular system; and connective 

 tissue corpuscles are found in the fibrous tissue which forms its ground- 

 work. The inner, looser, or sub-pleura! tissue contains lamellae of fibrous 

 connective tissue and connective-tissue corpuscles between them. Nu- 

 merous lymphatics are to be met with, which form a dense plexus of 

 vessels, many of which contain valves. They are simple endothelial 

 17 



