146 



THE ESSENTIALS OF HISTOLOGY. 



The cartilages of the trachea and larynx are hyaline, except the 

 epiglottis and the cartilages of Santorini and of Wrisberg, which are 

 composed of elastic fibro-cartilage. 



The lungs are formed by the ramifications of the bronchial tubex 

 and their terminal expansions, which form groups of sacculated dila- 

 tations (infundibula), beset everywhere with small hemispherical 

 bulgings, known as the air-cells or pulmonary alveoli. 



The bronchial tubes (figs. 168, 169) are lined in their whole extent 

 by ciliated epithelium which rests on a basement membrane. External 

 to this is the corium of the mucous membrane, containing a large 

 number of longitudinal elastic fibres and some lymphoid tissue. Out- 

 side this again is a complete layer of plain muscular fibres encircling 

 the tube. Next comes a loose fibrous layer in which, in the larger 

 tubes (fig. 168), small plates of cartilage are embedded. Mucous 

 glands are also present in this tissue. 



cL c O 



FIG. 169. SECTION OF A SMALL BKONCHIAL TUBE FROM THE PIG'S LUNG. 

 (F. E. Schultze.) (This section is much more magnified than that represented 

 in the previous figure.) 



a, fibrous layer ; 6, muscular layer ; c, mucous membrane in longitudinal folds, with numer- 

 ous longitudinally running elastic fibres cut across ; d, ciliated epithelium ; /, surround- 

 ing alveoli. 



The smallest bronchial tubes, which are about to expand into the 

 infundibula, gradually lose the distinctness of the several layers, their 

 wall at the same time being greatly thinned out and becoming bulged 

 to form the alveoli. The epithelium also becomes changed ; from 

 columnar and ciliated it becomes cubical and non-ciliated. 



In the alveoli themselves, besides small groups of cubical cells 

 there are large irregular flattened cells (fig. 170), which form an 

 extremely delicate layer, separating the blood-capillaries from the air 



