AN'ATOMT 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 : tlie 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 stej) is to 

 seize the posterior end of the sternum with a pair of for- 

 ceps, lift it up, cut the fibres wJiicli 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 

 it entirely. Underneath the sternum lies a thin-walled bag, 

 the perica?xlmin, 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 oesophagus and drawn 

 aside together with the intestine, while the liver must be 

 turned over 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 shajDC ; 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 tnmciis a7-ieriosi(,s (Ao), whic"h 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 ( F) passes from the liver to the back of the heart, and 

 there empties into a thin-walled sac, the suitis venosus, 

 which also receives on either side a vein from above, the 

 ve?im cava sujjeriores. The vein from the liver receives also 

 the genital and renal veins, and is then called the vena cava 

 inferior. As the heart continues to beat for many lionrs 

 after a frog has been killed, if a fresh si)ecimeu is taken for 

 dissection the rythmically alternating dilatations and con- 

 tractions may be observed. The order of contraction is, 



