XXVII ] 



THE RESPIRATORY ORGANS. 



299 



(c.) (H) In each alveolus stained oval nuclei, belonging to the 

 squamous epithelium lining it, but the outline of the squames them- 

 selves cannot be seen. Sections of the capillaries on the wall of the 

 alveolus and their nuclei stained. 

 In the thin walls, separating ad- 

 joining alveoli, fine elastic fibres, 



6. Respiratory Epithelium 

 of the Alveoli. (i.) Remove 

 the lungs from a young kitten, 

 and fill them with a \ per cent, 

 solution of silver nitrate. Run 

 in the fluid from a pipette pro- 

 vided with a bulb on its stem. 

 Pump the lungs from time to 

 time to remove as much air as 

 possible. After half an hour 

 replace as much as possible of 

 the silver solution by means of 

 alcohol. Tie the trachea, and 

 suspend the lungs in alcohol till 

 they are hardened. Cut sections 

 by freezing, mount in Farrant's 

 solution, and expose the sections 



to light. They become brown. Fia 

 Sections can be stained in 

 hsematoxylin or picro-oarmine. 



(ii.) Fill the lungs with .25 per cent. AgN0 3 , and suspend them 

 in .5 per cent. AgN0 3 (1-2 hours). Diffusion takes place. Cut 

 into pieces and place in alcohol (80 per cent.). 



(H) Select an alveolus which is so divided as to have a base, and 

 observe the silver lines showing the boundary-lines of the squamous 

 epithelium which lines it. At the junction-points of some of these 

 are groups of two or three small, polyhedral, granular cells stained 

 deep brown (fig. 288). 



7. Blood-Vessels of a Mammalian 

 Lung. Mount in balsam sections of an 

 injected lung. 



(L and H) Observe the alveoli, each 

 beset with a dense plexus of capillaries 

 (fig. 289), and especially where the edge 

 of an alveolus is seen, the wavy course 

 of the capillaries passing from one side 

 to the other of the inter-vesicular septa. 

 Search for the termination of a branch of 

 the pulmonary artery and the commencement of the pulmonary vein. 



288. Alveoli of Lung of Kitten, 

 Silvered, a, b. Squames ; d. Granular 

 cells ; c. Young epithelium-cell ; e. 

 Alveolar wall. 



FlG. 289. Capillaries of Human 

 Lung, Injected, x 90. 



