122 THE ESSENTIALS OF HISTOLOGY 



Outside 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. 146), small plates of cartilage are embedded. Mucous 

 glands are also present in this tissue. 



Fio. 147. SECTION OF A SMALL BKONCHIAT, TUBE FROM THE PIG'S LUNG. 

 (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. 148) which form an 

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

 within the alveoli. The capillary network of the alveoli is very 

 close (fig. 149), and the capillary vessels of adjoining alveoli are in 

 complete continuity, the vessels passing first to one side and then to 

 the other of the septa which separate the adjacent alveoli. 



Blood-vessels. Branches of the pulmonary artery accompany the 

 bronchial tubes to be distributed to the capillary networks upon the 

 alveoli, from which the blood is returned by the pulmonary veins 

 which, pursuing a separate cob^se through the tissue of the lung, join 

 in their course with others to K?rm larger vessels which pass to the 

 hilus. Branches from the bronchial arteries are distributed to the 

 walls of the bronchial tubes, and to the connective tissue of the lung. 

 This tissue intervenes everywhere in small quantity between the in- 

 fundibula (interstitial tissue), and forms a distinct layer, containing 

 much elastic tissue, covering the surface of the lung underneath the 



