302 



HISTOLOGY 



alveoli, but merely surround the main shaft of the duct, the layer is greatly 

 interrupted, and some consider that it ends in the course of the duct. 

 The respiratory bronchiole may be continued as a single alveolar duct or 

 may divide into two or more. The alveolar ducts branch to produce ahe- 



Pores. Cuboidal epithelial cells. Non-nucleated 

 \ ,__JbtfMUb Plates. 



Cuboidal 



epithelial Non-nucleated 

 cells. plates. 



Border of an alveolus. B Fundus of an alveolus. 

 FIG. 297. FROM SECTIONS OF THE HUMAN LUNG. X 240. 



A, Mixed epithelium of a respiratory bronchiole; B, an alveolus sketched with change of focus; the border 

 of the alveolus is shaded; it is covered by the same epithelium as that of the (clear) fundus of the 

 alveolus; the nuclei of the cells are invisible. (Silver nitrate preparation.) 



olar sacs (infundibula) which are cavities in the center of clusters of alveoli. 

 The sacs resemble the ducts as shown in Fig. 296. 



According to Miller (Arch. f. Anat. u. Entw., 1900, pp. 197-228) who has made 



careful reconstructions of the terminal 

 branches in the human lung, an atrium, or 

 round cavity, should be recognized between 

 the alveolar duct and the alveolar sac. The 

 alveolar duct is said to terminate by open- 

 ing into 3 to 6 atria, the entrances to which 

 are surrounded by smooth muscle fibers 

 forming "a sort of sphincter"; the atria pos- 

 sess no muscle fibers. Each atrium is con- 

 nected with two or more alveolar sacs, and 

 is moreover beset with alveoli (Fig. 298). 

 Stohr states that the recognition of an 

 atrium between the alveolar duct and 

 alveolar sac seems to him superfluous; "in 

 good casts of the human lung it is not to 

 be distinguished, and in other animals it is 

 inconstant." 



FIG. 298. CAMERA LUCIDA DRAWING FROM A 

 SECTION OF A CALF'S LUNG. (Miller.) 



The stippling indicates smooth muscle and cu- 

 boidal epithelium ; the lines, respiratory epithe- 

 lium. B. R., Respiratory bronchiole; D. A., 

 alveolar duct; A., atrium; A. S., alveolar sac. 



In sections, without resort to reconstructions, very little can be found 

 out concerning the relations of the alveoli to the bronchial ramifications. 

 The following structures are all which can easily be identified: (i) alveoli; 



