THE LUNGS. 609 



and a half long, half an inch wide, and situated upon the 

 internal surface of the lung, a little above its centre. It 

 fixes the lung on either side. 



The bronchi, or air tubes, (Fig. 165,) form the terminat- 

 ing branches of the trachea, coming off about opposite the 

 fourth dorsal vertebra, and consisting of the right and 

 left bronchus. 



The right passes beneath the right pulmonary artery to 

 the lung, is about an inch long before dividing, and is 

 larger and shorter than the left. 



The left bronchus is about an inch longer than the right, 

 though smaller, and passes through the arch of the aorta 

 obliquely downward to the left lung. 



The right bronchus is embraced at the root of the lung 

 by the vena azygos ; the left by the arch of the aorta. 

 The right, on entering the lung, divides into three branches, 

 the left into two, and both right and left then divide and 

 subdivide into an almost infinitude of branches throughout 

 the lungs. At the bifurcation of the trachea into the 

 bronchia, there is observed a triangular ligament, strong 

 and elastic, occupying the space of separation ; and after 

 entering the lung, the primitive divisions of each bron- 

 chus divide into two, and each one of these again into 

 two, and so on dichotomously as far as they can be traced, 

 the fine tubes ultimately terminating in the lobules, which 

 latter compose the air-cells, and these again consist sim- 

 ply of the dilated terminations of the extended branch- 

 ings of the bronchial tubes throughout the pulmonary 

 structure. 



Each bronchus, in its primitive division, like the trachea, 

 has its cartilaginous rings deficient in the posterior third. 

 But on entering the lungs, the rings form smaller seg- 

 ments of circles, and consist of small pieces placed equally 

 round the bronchial tubes so as to constitute them cylin- 

 ders. These pieces have different forms, and can overlap 

 and glide upon each other, by means of the circular mus- 

 cular coat, whose fibres are connected with the extremities 

 and margins of these cartilages, and in this way the diam- 

 39 



