102 



PRACTICAL MICKOSCOPY. 



The epithelial cells lining the bottom are obscured by the opaque 

 capillaries, and show only between the loops. It is probable that 

 these cells cover the plexus completely as they line the alveoli. 



We now encounter an obstacle which will frequently be met in our 

 study of organs. It consists of the difficulty in recognizing in sec- 

 tions the plan of structure which we have learned is peculiar to the 

 organ under consideration. For example: A lung has been compared 

 to a tree. The bronchi are the representatives of the branches, and 

 the air sacs of the fruit. Well, we make a section from human lung 

 it matters little as to the direction with every possible care, and 



FIG. 74. TRANSVERSE SECTION OF A SINGLE PULMONARY ALVEOLUS. 

 Stained with Hsema. and Eosin. X 400. 



Capillaries injected. 



A, A, A. Walls of the alveolus. 



B, B. Injected capillaries. 



C, C. Pavement cells lining the alveolus. These cells cover the capillaries, but do not so ap- 

 pear in the drawing, as the latter are filled with an opaque injection. The observer is supposed 

 to be above the sectioned alveolus, viewing the cup-shaped cavity. 



the image in the neld of the microscope resembles a fragment of rag- 

 ged lace more nearly than anything else ! The arrangement of the 

 tubes and alveoli of the lung has been determined by filling the cavi- 

 ties with melted wax which, when cold, and the tissue destroyed by 



