CHAP, xxix.] THE HUMAN LUNGS. 387 



fibres, in the course of which we have discovered unstriped musciu 

 lar fibres to occur, especially about the bifurcation. In this fibrous 

 layer are recesses for the tracheal glands, and on dissecting it off 

 these glands are exposed, together with a thin sheet of transverse 

 unstriped fibres, completing as it were, the circle of the carti- 

 laginous rings, and known as the trachealis muscle. The fibres of 

 this are attached a little way from the extremities of the cartilages 

 on their inner surface, and in contraction must serve to approxi- 

 mate them, and thus to narrow the canal. In the horse, they 

 are inserted three-fourths of an inch from the extremities, which 

 almost or quite overlap. In birds they are composed of striped 

 fibres. 



The cartilages, their interspaces, and the trachealis muscle are 

 lined by a thin layer of longitudinal elastic anastomosing fibres, 

 uniformly spread out except over the trachealis muscle, where they 

 are gathered up into longitudinal bands, sometimes one-twelfth of 

 an inch thick, very visible through the mucous membrane. These 

 take a serpentine course down the bronchi, and preserve their 

 anastomosing character. The trachea owes its elasticity in the 

 longitudinal directions to the fibres now described. 



The mucous lining of the trachea, the essential part of the duct, 

 to which the above are accessory, is continuous through the glottis 

 with that of the pharynx, and physiologically with the respiratory 

 compartment of that cavity, and with the nasal passages (see ante 

 p. 185). It is covered with ciliated epithelium, and the direction 

 of the movement is probably upwards towards the glottis. The 

 tracheal glands are productions of this membrane, and appear as a 

 layer of reddish, distinct granules, behind the trachealis muscle, 

 each one being furnished with a separate duct, traversing first the 

 muscle and then the layer of longitudinal elastic fibres, to open on 

 the inner surface of the tube. The glands appear to be tubular^ 

 not follicular, and are thus related rather to the sudoriferous than 

 to the salivary system. They probably furnish much of the halitus 

 of the breath and may determine its odour. 



Of the bronchi or primary subdivisions of the trachea, the right 

 is the shorter, wider, and more horizontal ; the left longer to pass 

 under the aortic arch. Their walls resemble those of the trachea 

 with slight modifications. At the root of the lung, each breaks up 

 into branches corresponding to the lobes (lobal broncjiia), and these 

 again into secondary, tertiary, and terminal bronchia, the last 

 named being from one-fortieth to one-sixtieth of an inch in diame- 

 ter. The terminal bronchia pass to portions of the pulmonary 



