7.6 



ZOOLOGY. 



all serous membranes, besides investing more or less com- 

 pletely the exterior of the organs, also line the walls of the 

 thorax, thus providing for the security and mobility of the 

 organs themselves. 1 * The lungs are two in number, com- 

 municating with the exterior by means of a single air- 

 tube, the trachea (b, Fig. 

 58), which ascends through 

 the fore part of the neck, and 

 opens into the pharynx. 



This canal or tube is 

 formed of a series of small 

 cartilaginous bands, incom- 

 plete behind. It is lined by 

 a mucous membrane, which 

 is of the same nature as the 

 mucous membrane of the 

 mouth and nostrils, and is 

 continuous with it. f Finally, 

 inferiorly, the trachea sub- 

 divides into two branches 

 called bronchi, which ramify 

 in both lungs like the roots 

 of a tree in the soil (c e, 

 Fig. 58). 



136. The lungs show 

 internally a vast number of 

 small cells, into each of 

 which a branch of the cor- 

 responding bronchus opens. 

 A soft, delicate, and vas- 

 cular membrane lines the walls of these cells and air tubes, 

 and it is to these that the alternate branches of the pulmonary 



Fig. 58. Lungs and Trachea 

 in Man.J 



* The pleurae are arranged, in fact, like other serous membranes. 



t The surface of the membrane of the trachea is covered with a fine down, 

 each hair of which exhibits vibratile movements, called also ciliary. This 

 vibratile movement determines in the liquid in contact with their surface, 

 currents, which are often very rapid, and which persist even after a portion 

 of the membrane has been removed from the body. The cilia are micro- 

 scopic. The direction of the current seems to be from the exterior towards 

 the interior, and the same movements may be observed on the surface of 

 tho nasal fossae, but nothing of the kind is to be seen in the pharynx. 



J One of the lungs (the left) is left in its natural condition (d) ; but in the 

 other the substance has been destroyed, in order to expose the right bron- 

 chus and its ramifications in the lung. $ 



a, larynx and superior extremity of the trachea ; b, trachea ; c, division 

 into bronchi ; d> one of the lungs ; e, bronchial ramusculee. 



