APPENDIX. 
407 
foot. Procure a thin board large enough to lay the frog upon ; 
in one end make a hole about a half-inch in diameter, over which 
the web may be stretched ; anaesthetize the frog with ether or 
chloroform ; as soon as the animal becomes insensible lay it on 
the board, with its body covered with a moist cloth ; over the 
larger toes of the foot to be examined slip nooses of thread, and 
fasten these in slits around the edge of the board in such posi¬ 
tions as to spread the web between two of the toes over the 
hole in the board. Put a drop of water on the web, lay on the 
cover-glass, place the board on the microscope, and examine 
with a one-fifth or a one-sixth objective. The anaesthetic must 
be renewed from time to time, otherwise the struggles of the 
animal will interfere with observation (Fig. 66). 
CHAPTER XIV. 
The gross structure of the frog’s lung may be studied in 
specimens which have been removed from the body, inflated 
with air blown through a small glass tube inserted through the 
glottis, and placed in alcohol a few hours to harden. When 
cut open the lung will be seen to be a hollow sac with corru¬ 
gated walls (Fig. 85). 
“ Plucks ” obtained from a butcher will illustrate the struct¬ 
ure of the mammalian larynx, trachea, bronchial tubes, etc. If 
fresh and not punctured with the knife they may be inflated. 
To work well they should be kept moistened (Fig. 86). 
The presence of carbon-dioxide in the air exhaled from the 
lungs may be shown by using lime-water or baryta-water, with 
either of which carbon-dioxide forms an insoluble precipitate, 
which at first floats as a delicate white film on the surface of 
the liquid. Pour some of the fluid into a saucer or watch-glass, 
then breathe heavily upon it a few times through the mouth, 
and the film will be formed. 
CHAPTER XV. 
The structure of the kidneys is well illustrated by the kid- 
