ANATOMY OF THE COMMON FROG. 471 
Slit open the skin along the median ventral line the 
whole length of the animal, turn the skin back, and then 
cut through the muscular walls of the abdomen, being care- 
ful not to injure the underlying organs. The viscera will 
then be exposed : the coiled intestine, the large liver, and in 
the female the sexual organs at either side; finally, pos- 
teriorly, the thin-walled bladder, B. The next step is to 
seize the posterior end of the sternum with a pair of for- 
ceps, lift it-up, cut the fibres which run from its under sur- 
face, and cut with a pair of strong scissors along both sides 
of the sternum and around its anterior end, so as to remove 
itentirely. Underneath the sternum lies a thin-walled bag, 
the pericardium, enclosing the heart. On either side are 
the lungs. 
To complete the preparation dissect out the intestine, by 
cutting through the mesentery ; follow it to the stomach, 
which must be separated from the cesophagus and drawn 
aside together with the intestine, while the liver must be 
turned oves to the right of the animal. The pericardium 
must be cut through and removed without injury to the 
heart; finally, the skin must be removed from the hind 
legs. If the dissection is of a male, it will then appear very 
much as in the figure. 
The heart is conical in shape ; its apex points backwards, 
and is formed by a single chamber, the ventricle, with thick 
muscular walls, from which springs on the ventral surface a 
little to the right the ¢runcus arteriosus (Ao), which runs 
forward and divides into the two aortic arches. The base of 
the heart contains two chambers, the right and left auricles, 
the separation of which is not marked externally. <A large 
vein (V) passes from the liver to the back of the heart, and 
there empties into a thin-walled sac, the sinus venosus 
which also receives on either side a vein from above, th 
vene cave supertores. The vein from the liver receives alse 
the genital and renal veins, and is then called the vena cava 
inferior. As the heart continues to beat for many hours 
after a frog has been killed, if a fresh specimen is taken for 
dissection the rythmically alternating dilatations and con- 
' tractions may be observed. The order of contraction is, 
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