360 RESPIRATION. 



seemed to show that the presence of the epiglottis, in the 

 human subject at least, is necessary to the complete protec- 

 tion of the air-passages in deglutition. 1 



Passing down the neck from the larynx toward the lungs, 

 is a tube, from four to four and a half inches in length, and 

 about three-quarters of an inch in diameter, which is called 

 the trachea. It is provided with cartilaginous rings, from 

 sixteen to twenty in number, which partially surround the 

 tube, leaving about one-third of its posterior portion occupied 

 by fibrous tissue, mixed with a certain number of unstriped 

 muscular fibres. Passing into the chest, the trachea divides 

 into the two primitive bronchi ; the right being shorter, 

 larger, and more horizontal than the left. These tubes, pro- 

 vided, like the trachea, with imperfect cartilaginous rings, 

 enter the lungs, divide and subdivide, until the minute rami- 

 fications of the bronchial tree open directly into the air-cells. 

 After penetrating the lungs, the cartilages become irregular, 

 and are in the form of angular plates, which are so disposed 

 as to completely encircle the tubes. In tubes of very small 

 size, these plates are less numerous than in the larger bronchi, 

 until in tubes of a less diameter than -^ of an inch, they are 

 lost altogether. 



The walls of the trachea and bronchial tubes are com- 

 posed of two distinct membranes: an external membrane, 

 between the layers of which the cartilages are situated, and a 

 lining mucous membrane. The external membrane is com- 

 posed of inelastic and elastic fibrous tissue. Posteriorly, in 

 the space not covered by cartilaginous rings, these fibres are 

 mixed with a certain number of unstriped or involuntary 

 muscular fibres, which exist in two layers : a thick internal 

 layer, in which the fibres are transverse, and a thinner longi- 

 tudinal layer, which is external. This collection of muscular 

 fibres is sometimes called the trachealis muscle. Throughout 



1 This remarkable case, in which the epiglottis had sloughed entirely away 

 leaving the parts completely cicatrized, as demonstrated by a laryngoscopic exam- 

 ination, will be given in extenso in connection with the subject of deglutition. 



