64 THE. FROG 
vapor is irritating to the eyes, and the solution may attack the 
fingers causing severe poisoning of the skin; and (b) frogs pre- 
served for a long time in formalin are almost worthless for the 
preparation of skeletons, since the calcareous material of the 
bones is gradually dissolved, leaving them soft and pliable. 
V. ANATOMICAL PREPARATIONS. 
It is frequently desirable that the student should see a model 
preparation of a system of organs as a guide for his own dis- 
section, or for the demonstration of features difficult for a begin- 
ner to make out. Moreover, as in the case of the skeleton, time 
and material will be saved by the use of permanent preparations 
that may be studied by successive classes. At least one example 
of each of the preparations described in this section should be 
provided, and it will be profitable to supply as many as the class 
can use. 
Wet preparations should be kept for exhibition in 5 per cent. 
formalin or 70 per cent. alcohol; in some cases, which will be 
pointed out, the alcohol has decided advantages. The jars should 
be of clear glass, tall enough to permit of some evaporation of 
the liquid before the preparation is exposed. Glass tops, either 
clamped on over rubber rings or with edges ground and made 
air tight by smearing with vaseline, are desirable, since metal is 
attacked by formalin and cork will discolor the alcohol. Each 
preparation should be provided with an appropriate label. 
A. The Digestive System.—An entire frog should be prepared 
with the tongue protruded. This may be accomplished by inject- 
ing the lymph space beneath the tongue of a fresh frog with warm 
gelatin prepared as follows: soak the dry gelatin in distilled 
water until it is swollen and soft (about 18 hours), then remove 
to a porcelain evaporating dish and melt at a temperature of about 
45° C. The injection causes the tongue to protrude as in the act 
of seizing food; the pressure should be kept up until the gelatin 
hardens, when the tongue will be kept permanently in this posi- 
tion. The frog should then be hardened in formalin and ex- 
hibited in formalin or in alcohol in a glass jar. 
In the study of the digestive system the bile duct and its rela- 
