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HANDBOOK OF PHYSIOLOGY. 



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 the an- 

 terior and internal surfaces of the lungs, the parts which move most 





Fig. 205. Ciliary epithelium of the human trachea, a, Layer of longitudinally arranged elastic 

 fibres ; b, basement membrane ; c, deepest cells, circular in form ; d, intermediate elongated cells ; 

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



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 lob- 



Fig. 206. 



Fig. 207. 



. Fig 206. Terminal oranch of a bronchial tube, with its inf undibula and air-cells, from the mar- 

 andaii5tlis nS X ^ ' ^^ 11 quicksilver ' a ' Termin al bronchial twig ; b 6, inf undibula 



Fig. 207. Two small infundibula or groups of air-cells, a a, with air-cells, b 6, and the ultimate 

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



ules. Each pulmonary lobule may be considered to be a lung in minia- 

 ture, consisting, 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. 



