RESPIRATION. 197 



one portion of the sac adhering closely to it, and one lying over 

 this again ; the internal surfaces of both portions are always in 

 contact, because, if the parietes of the thorax expand and draw 

 with them the external portion, the lung at the same time expands 

 with air and forces forwards the internal in the same degree. 



The union of the two pleurae, from the sternum to the spine, is 

 called the septum or mediastinum. The heart lies between the two. 

 Before the two unite at the posterior part, they leave a cavity, 

 called the cavity of the posterior mediastinum, containing the 

 aorta, oesophagus, thoracic duct, vena azygos, large bronchiae, lym- 

 phatic ganglia, and cellular membrane. In front, they leave an- 

 other space, called the cavity of the anterior medastinum, which 

 contains cellular membrane, and in the foetus the thymus gland. 



" The lungs hang, in a manner, from the wind-pipe, usually 

 called aspera arteria or trachea," " which is composed of an in- 

 ternal mucous membrane continuous with that of the fauces, of 

 an external fibrous membrane, and of from fifteen to twenty fibro- 

 cartilaginous falciform arches, imperfect at the posterior part, 

 where transverse muscular fibres connect the two extremities of 

 each cartilage. Within this muscular coat, and throughout the 

 trachea and bronchiae, a coat of longitudinal fibres is seen. 



" The aspera arteria, having entered the thorax, is bifurcated " 

 (the right branch the shorter and wider) " opposite the third 

 dorsal vertebra," " into the trunks of the bronchiae, and these, the 

 more deeply they penetrate into the lobes and lobules of the lungs, 

 are the more and more ramified;" the fibro-cartilaginous rings are 

 each divided into pieces, more and more numerous and smaller, 

 till they are mere grains, and at length are lost, together with the 

 external fibrous coat ; and the extreme divisions, consisting of 

 the mucous membrane, and probably of the circular longitudinal 

 fibres immediately external to it, terminate in those cells which 

 form the chief part of the substance of the lungs and alternately 

 receive and emit the air we breathe. 



" The shape and magnitude f of the air-cells are various. The 

 former is generally polyedrical. The latter, in regard to surface, 

 is scarcely to be defined 2 : though, indeed, the capacity of the 

 lungs of an adult, during a strong inspiration, is about 120 cubic 



f " Keil, indulging his luxuriant iatro-mathematical genius, assigned more 

 than 1,744,000,000 cells to each lung." 



e " Lieberkiihn, with equal exaggeration, made the surface of the cells equal 

 to 1500 square feet." 



