THE FINER STRUCTURE OF THE LUNG 283 



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

 them is only just as much fluid as will insure frictionless movement in their 

 expansion and contraction. 



When considering the subject of normal respiration, one 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 considerable 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 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. 



The pulmonary pleura consists of an outer or denser layer and an inner 

 looser tissue in which there is a lymph-canalicular system. Numerous 



FIG. 222. Transverse Section of the Chest. 



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 fiber occur in the pulmonary pleura. They are es- 

 pecially strongly developed on the anterior and internal surfaces of the lungs, 

 the parts which move most freely in respiration. Their function is doubt- 

 less to aid in expiration. 



The Finer Structure of the Lung. Each lung is partially subdi- 

 vided 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 num- 

 ber of minute parts, called lobules. Each pulmonary lobule may be con- 

 sidered to be a lung in miniature, consisting, as it does, of a branch of the 

 bronchial tube, of air-cells, blood vessels, nerves, and lymphatics, with a 

 small amount of areolar tissue. 



