242 



HISTOLOGY. 



FIG. 273. CAMERA LUCIDA DRAWING FROM A 

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



The stippling indicates smooth muscle and cuboid- 

 al epithelium; the lines, respiratory epithe- 

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

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



which several alveolar sacs proceed (Fig. 273). If the student in examining 

 this figure questions why the atria are not alveolar ducts, and the alveolar 

 ducts are not respiratory bronchioles, it may be said that these terms are 

 variously employed by different histologists, and that atria are not recog- 

 nized by German writers. It seems questionable that the final ramifi- 

 cations of the lung are so definitely 

 arranged as to justify the cumber- 

 some nomenclature in current use. 

 Fig. 273 shows, however, exactly 

 what may be expected in any sec- 

 tion of the lung, namely (i) alveoli; 

 (2) spaces bounded by alveoli 

 (alveolar sacs, atria, alveolar ducts, 

 the last being supposed to have mus- 

 cle fibers associated with them) ; (3) 

 small bronchioles with alveoli along 

 their walls, therefore consisting of 

 a mixed epithelium (respiratory 

 bronchioles); and (4) bronchioles 

 with no respiratory epithelium. 



The alveolar walls have been 



described as consisting of respiratory epithelium (Fig. 272, B). The 

 non-nucleated plates are presumably derived from the flattened nucleated 

 cells scattered among them, and large plates arise from the fusion of small 

 ones. In amphibia, nuclei in small amounts of protoplasm are found 

 attached to the edges of the plates, and projecting into the connective 

 tissue between the capillaries 



The abundant capillary network _ *-**^ - Vein. 



of the alveolar walls is shown in 

 Fig. 274; lymphatic vessels are 

 absent. Elastic tissue is highly 

 developed around the alveoli and 

 forms rings encircling their out- 

 lets. In inspiration an alveolus 

 may expand to three times the 

 diameter to which it returns dur- 

 ing expiration (o.i to 0.3 mm.). 



Pores have been described, leading from one alveolus to another (Fig. 

 272,6). 



The pleura is essentially similar to the peritonaeum, consisting of a 

 connective tissue layer covered with a flat epithelium (mesothelium). Per- 



Capillaries. 



Artery. 



FIG. 274. FROM A SECTION OF THE LUNG OF A 

 CHILD, INJECTED THROUGH THE PULMONARY 

 ARTERY. X 80. 

 Of the five alveoli drawn the three upper ones are 



fully injected. 



