THE LUNGS. 199 



and through with the cacao-butter, is placed in the 

 pit in a position convenient for making sections, and 

 the pit is then filled with the melted cacao-butter. 

 This as it hardens adheres firmly to and sets into 

 one piece with the cake. When quite hard a pro- 

 cess which takes a considerable time sections of 

 the tissue are made with a razor wetted with spirit. 

 Since all the cavities are filled by and the tissue is 

 thoroughly impregnated with the cacao-butter, the 

 whole cuts like a homogeneous piece of this substance, 

 and sections can be made as thin as desired. As they 

 are cut they are placed in oil of cloves, which removes 

 the cacao-butter in a few minutes, even in the cold ; 

 but in the winter season it is better to accelerate the 

 solution by slight warmth. The sections can then 

 be mounted in dammar. 



These stained sections of lung may be first ex- 

 amined with a moderate power, but afterwards a 

 power of 400 or 500 diameters should be employed, 

 in order to see the details of structure ; the ciliated 

 epithelium, muscular layer, and cartilaginous plates 

 of the bronchial tubes, with the mucous glands, 

 nerves, lymphatics (seen in section as mere clefts), 

 and patches of lymphoid tissue in their walls ; the 

 branches of the pulmonary artery accompanying 

 them ; the mode in which the terminal air-tubes 

 dilate into the infundibula ; the air-cells or alveoli, 

 almost covered with a network of capillaries which 

 are seen also on the septa between the alveoli, pro- 

 jecting fii;st into one and then into the other of two 

 neighboring air-cells. Where they run vertically the 

 capillaries appear in optical section as circular spots, 

 looking not unlike nucleated cells. But the exces- 

 sively delicate epithelium of the air-cells cannot be 

 well seen in these preparations, for the epithelium 

 cells remain almost unstained, and it is not easy to 

 differentiate their nuclei from those of the closely 

 subjacent capillaries. 



Preparation 6. Epithelium lining the air- 

 cells. In order to demonstrate the epithelium cells 



