THE LUNGS 125 



diameter of about one millimeter has been reached. The cartilage is 

 of the hyaline variety; and each plate is covered with a dense fibrous 

 coat, the perichondrium , which unites it with contiguous parts. 



The principal bronchi are provided with a great number of 

 mucous glands, which are located in the external coat or submu- 

 cosa. They are simple, coiled tubular glands, commencing on the 

 inner surface, penetrating the mucosa and muscularis mucosae, 

 and terminating in the submucosa, generally within the cartilage, 

 where they are coiled in short, close turns in sections resembling 

 somewhat the larger sweat-glands of the skin. The ciliated epi- 

 thelium of the bronchial tube is continued down the beginning of 

 the tube for a short distance, after which the cells are shortened, 

 and lose their cilia. The coiled gland-part of the tube is lined with 

 conical cells, which are so large as to leave the lumen very small. 

 Sometimes, and especially in the aged, an ampulliform dilatation' 

 of the tube may be seen during its passage through the mucosa. 



The description just given will apply to large and medium- 

 sized bronchial tubes. Very important changes take place as we 

 pass to the terminal tubes. 



As the tubes decrease in size, the first coat to diminish in 

 thickness is the outer, or submucosa. We have already alluded to 

 the disappearance of the cartilage, and the mucous glands are 

 lost at about the same time. The outer coat becomes, in the 

 small bronchial tubes, so thin as to be no longer distinctly demon- 

 strable. The muscular coat is the last to disappear. It remains 

 a prominent feature of the tube as long as separate coats can be 

 distinguished. The epithelial cells lining the tubes toward the 

 termini become shortened, and, getting lower and lower, at last 

 result in cuboidal cells, without cilia. 



The walls of terminal bronchial tubes (diameter one -fourth 

 to one -eighth of a millimeter) are composed of a slight amount of 

 connective tissue in which an occasional non- striated muscle -cell 

 and yellow elastic fiber can be distinguished. They are lined 

 with cuboidal or a few flat cells. No definite layers are distin- 

 guishable in these bronchial tubes. In a transverse section the 

 lumen would appear circular. 



PRACTICAL DEMONSTRATION 



The histology of the bronchi can be studied to best advantage by using 

 tissue from a freshly killed pig, cat, or dog. Short pieces of tubes, about one 

 centimeter in diameter, from which most of the lung- substance has been cut 



