XX.] THE STRUCTURE OF THE LUNG. 217 



will be seen best where a portion of the 

 wall of the alveolus is left flat in the 

 section; sometimes it may be seen in 

 profile. 



7. The elastic fibres of the alveolar wall. 



. The nuclei and collapsed walls of the 

 capillaries. 



7. Take a piece of mammalian lung the blood- 

 vessels of which have been injected. Treat as in 

 o but without staining. 



Observe the close-set capillary net-work over the 

 alveolus, and the artery forming a circle round 

 the mouth of each alveolus. 



DEMONSTRATIONS. 



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



2. Tracing on the revolving drum of the movements 

 of the column of air in respiration. 



the thorax punctured on each side (or the chest walls removed) and 

 the lungs distended with a *2 p.c. solution of silver nitrate; by 

 alternately forcing the fluid in and drawing it out of the lungs the 

 air will be gradually displaced by the silver solution ; when this is the 

 case, 30 p.c. alcohol is several times injected into and drawn out of 

 the lung so that the nitrate of silver is displaced by 30 p.c. alcohol, 

 this is then displaced by 50 p.c., and so on up to strong spirit with 

 which the lungs are distended ; the trachea is then tied, the lungs cut 

 out and placed in strong spirit. Sections (best cut with a freezing 

 microtome) are exposed to light and treated as usual. 



