XXI V] LUNG 191 



somewhat short primary septa running inwards from 

 the wall of the lung, and forming a number of polygonal 

 chambers open towards the central space, (c) short 

 secondary septa, projecting into the chambers from the 

 primary septa. 



5. Section of newt's lung (distended with Flem- 

 ming's fluid; picrocarmine). Note the epithelium cells, 

 the thicker part between the capillaries containing the 

 nucleus, the thin expansion of the cell lying over an 

 adjoining capillary. 



6. Section of mammalian lung treated with nitrate 

 of silver to show the epithelium cells of the alveoli. 



7. The circulation in the lung of toad or frog. 



NOTES. 



Outlines of flat cells of alveoli. Expose the lungs of a de- 

 capitated rat. Inject into the trachea a mixture containing 

 25 p.c. silver nitrate, and '5 p.c. osmic acid ; force the fluid 

 repeatedly in and out, so as to remove the air as far as possible 

 from the alveoli. In 15 minutes cut up the lungs, wash with 

 water ; place a piece in gum, cut it frozen, expose the sections to 

 light for \ hour ; mount in balsam. The other pieces may be 

 kept in 70 p.c. alcohol. 



Sections may be stained on the slide with a nuclear stain. It 

 is generally held that the large flat cells of the alveoli have no 

 nuclei ; owing to the curved form of the alveoli it is difficult to 

 make decisive preparations, but it is on general grounds unlikely 

 that non-nucleated cells should persist for any length of time ; 

 nuclei in division are rare in the lung cells. 



Circulation in lung of frog or toad. If the lungs collapse, 

 and they usually do in a frog, take a cannula with two necks, 

 about 4 mm. apart, and with a small hole in the glass between 



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