ANATOMY OP TEE COMMON FMOG. 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 fibred wliich 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 iKricdrdium, enclosing the heart. On eitiier 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 shape ; its apex points backwards, 

 and is formed by a single chamber, the ventricle, with thick 

 muscular walls, from which springs on the ventrcd surface a 

 little to the right the truncus 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 LirL'^e 

 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, the 

 vencB cavcB superiores. 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 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» 



