202 BIOLOGY 



The order of dissection given below is so planned as to make it pos- 

 sible to do practically all of the dissection upon a single specimen. The 

 specimen may be preserved in formalin and the work carried out at leisure. 

 If the order given is followed, it is possible to have a large class working 

 at the same time, and, when the work is finished, all of the important 

 parts in the anatomy will have been made out, except the skull and the 

 shoulder girdle, these having of necessity been destroyed in opening the 

 body and in exposing the brain. If frequent references are made to the de- 

 scription of the frog given in the text, the brief description here given 

 will be sufficient to make a satisfactory dissection. 



Open the frog by a median ventral incision, made with scissors, ex- 

 tending from the legs forward to the sternum, cutting through both skin 

 and flesh. The blunt end of the scissors is then to be thrust under the 

 sternum, and this girdle of bones is to be cut through. This will make 

 it possible to open the abdomen, pinning out the flaps of the abdominal 

 walls and the arms so as to expose the organs of the abdomen. If the frog 

 is a freshly killed specimen, all of the subsequent study of the viscera 

 should be made with the animal immersed in water. If the frog is a pre- 

 served specimen, this is not so necessary. 



The organs of the abdomen may now be studied. The following parts 

 should be made out without any further dissection, being disclosed simply 

 by pushing the organs one after the other to one side, and they may be 

 examined conveniently in the following order: liver; heart; large arteries 

 around the heart; veins entering the heart; stomach; intestine; gall blad- 

 der; rectum; mesentery, which contains blood vessels that may be 

 traced to the liver. 



In opening the body, if the specimen is a fresh one, there is danger 

 that some of the blood vessels may be cut, making it difficult or impos- 

 sible to follow the blood vessels. In order to work out the blood vessels 

 satisfactorily, it is necessary to have an injected specimen. These may 

 be bought of dealers in natural history supplies, or the injection may be 

 done by the instructor. 



If the specimen is a female, the body cavity will, at certain seasons of 

 the year, be filled with an enormously expanded ovary, filled with eggs. 

 In order to make out the other abdominal organs, these must be removed 

 carefully, so as not to injure the other parts. After they have been 

 removed there will appear lying on either side of the back part of the 

 abdomen the very much enlarged oviduct, showing as a much coiled 

 tube. This should also be removed, note being made of its connection 

 with the cloacal chamber behind. If the ovary is not thus enlarged, or 

 if the specimen is a male, it is not necessary to remove the reproductive 

 organs to show the other features. 



