STRUCTURE OF THE LUNGS OF MAMMALS. 53 



structurally distinct layers are distinguishable, independently of 

 the tunica adventitia, which is composed of a variable amount 

 of loose fibrillated connective tissue that contains here and there 

 small masses of fat, and serves to connect the bronchia with 

 the adjoining tissues, as with vessels, lymphatic glands, nerves, 

 and alveolar tissue. The outermost of these layers, which consti- 

 tutes more than half of the entire thickness of the wall, is the 

 external fibrous layer, chiefly composed of dense fibrillated 

 connective tissue, with imbedded cartilaginous plates. These 

 last, so important in conferring firmness and elasticity on the 

 bronchial tubes, still preserve in the primary branches, as in 

 the bronchia themselves, the form of flattened semi-circles; 

 they do not however, any more than in these, present a 

 posterior defective portion, closed only by membrane ; but, 

 distributed over every part of the tubes, form a peculiar frame- 

 work for supporting and strengthening them. 



Fig. 127. 



Fig. 127. A portion of the transverse section of a human bronchial 

 tube, having a diameter of six millimeters, magnified thirty diame- 

 ters, a, External fibrous layer ; 6, muscular layer ; c, internal fibrous 

 layer, with the hyaline basement membrane ; d, epithelial layer. 



To these semi-circular cartilages, the sharp borders of which 

 in many animals are in immediate contact with each other, or, 

 as in the Pig, even to some extent overlap, irregularly angular 

 plates provided with short processes quickly succeed in Man ; 

 though in large animals, as the Cow and Horse, these do not 

 appear until the bronchi have undergone several divisions. 



