THE LUNGS. 197 



the latter the silver solution is, as before, to be 

 allowed to remain a shorter time than on the brushed 

 part, where the fluid has to penetrate into the 

 lymphatic vessels, and into the substance of the tis- 

 sue. The whole thorax may then be cut off from 

 the rest of the trunk and exposed under water to the 

 light ; or, if it be too large to do this conveniently, 

 a piece only of the thoracic parietes on each side is 

 to be removed and pinned out on to a loaded cork, 

 which is then placed in a dish of water in the sun- 

 light. When stained pieces of the membrane must 

 be carefully dissected off, without pulling upon or 

 injuring the tissue in any way, floated upon a slide, 

 the excess of water poured off or soaked up with 

 blotting-paper, all creases removed from the mem- 

 brane, and finally the cover-glass superadded, with 

 a drop of glycerine. 



Preparation 4. The lung tissue. The struc- 

 ture of the lungs themselves is best shown by means 

 of sections. The tissue is hardened in the following 

 way: 



The organs having been removed from the chest 

 of a recently killed rabbit or cat, care being taken 

 not to scratch their surface with the broken ends of 

 the ribs, a glass canula is tied into the end of the 

 trachea (or into either bronchus). The canula is 

 then connected by an India-rubber tube with an in- 

 jection bottle, which is filled with a weak solution 

 of chromic acid ( per cent.). By blowing air into 

 the tube this solution is made to flow into the lungs 

 so as to distend them moderately. The trachea or 

 bronchus is now tied up, the canula removed, and 

 the lungs are immersed in a large quantity of a solu- 

 tion of chromic acid of similar strength. After two 

 days the fluid is changed, \ per cent, solution being 

 substituted, and the organs are cut into pieces, to 

 enable the fresh fluid more readily to penetrate. 

 After a week more in this the pieces are placed first 

 for twenty-four hours in weak spirit and then in 

 strong spirit. 



17* 



