THE FINER STRUCTURE OF THE LUNG 



249 



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 lym- 

 phatics 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, 



FIG. 223. 



FIG. 224. 



FIG. 223. Terminal Branch of a Bronchial Tube, with its Infundibula and Air-cells, from the 

 Margin of the Lung Injected with Quicksilver; Monkey, a, Terminal bronchial twig; b, b, in- 

 fundibula and air-cells. X 10. (F. E. Schulze.) 



FIG. 224. Two Small Infundibula, a, a, with air-cells, b, b, and the ultimate bronchial tubes, 

 c, c, with which the air-cells communicate. From a new-born child. - (Kolliker.) 



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 considered 

 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- 



