302 PULMONARY MUCOUS TISSUE. 



been violated ; being aided in this action by the elastic and 

 muscular tissues. 



The action of the muscles of the neck gives to the 

 trachea an ascending and descending motion, corresponding 

 with the respiratory movements. During inspiration the 

 trachea descends; its calibre increases, and the current of air 

 passes through easily, without friction. During expiration, 

 it rises, lengthens, and thus becomes narrower ; the channel 

 through which the air passes out, being narrower than that 

 through which it entered, causes the air to circulate more 

 rapidly, and increases friction against the sides. 



The larynx has also a large snare in producing the differ- 

 ence between the current of air which is inhaled and that 

 which is exhaled. When we study this organ as a vocal 

 apparatus, we shall find that it is composed chiefly of an 

 antero-posterior aperture (glottis), capable of enlarging and 

 of narrowing, enlarging in inspiration, and narroicing in ex- 

 piration. The degree of this narrowing differs in different 

 cases : when a person makes a muscular effort, as for instance 

 in defecation, the opening is entirely closed ; the air can 

 then no longer escape, and is compressed by the thorax, 

 which forms a point of support to the muscles which are con- 

 cerned in the effort. 



The object of the difference in the velocity of the cur- 

 rent of air when inhaled and exhaled, is the expulsion of 

 foreign bodies, or rather, of those mucosities which may be 

 found in the respiratory tree. The column of air, in inhala- 

 tion, passes too slowly and with too little friction to en- 

 able it to bring out the mucosities which adhere to the 

 wall ; the current of exhaled air, on the other hand, present- 

 ing the opposite conditions, drags these small collections of 

 matter forcibly to the upper orifice of the air-vessels. 



In coughing, the expiration is more sudden, and the pre- 

 ceding inspiration slower than the normal expiration and 

 inspiration ; the chief effect of coughing is thus to throw off 

 the mucosities which obstruct the respiratory or air tubes. 



This continuous and unconscious expulsion of the mucos- 

 ities is also effected by the movements of the vibratile cilia 

 with which the columnar epithelium of the entire bronchial 

 and tracheal tubes (except at the level of the vocal cords) 

 are furnished ; the movements of these cilia are of such a 

 character that they carry to the exterior all the little bodies 

 deposited on their surface, conveying them as far as to the 

 cavity of the larynx (see p. 190). It is here only that expul- 



