THE FROG 



THERE are many species of true frogs (Rana) in the United States, but 

 the following directions may be used with any; the larger the speci- 

 mens the better. 



For study each student should be provided with two specimens, a 

 male and a female, besides a prepared skeleton. One, preferably the 

 male, should have both arterial and venous systems injected. This should 

 be used for the general study of the viscera and the circulatory organs, 

 while the other should be used for the muscles, the internal structure 

 of the heart, the female urogenital organs, and the nervous system. 



The arterial system may be filled by inserting the canula in the bul- 

 bus cordis and ligating it in the truncus arteriosus impar. The venous 

 system is injected by forcing the injecting fluid in both directions through 

 the ascending portion of the abdominal vein, and forward and back 

 through the postcava. 



Skeletons may be prepared by skinning the frog, separating the fore 

 limbs and the supporting girdle, and heating for a short time in soap 

 solution. Then pull and brush away as much of the flesh as possible, 

 repeating the use of the soap solution if necessary. Care must be used 

 not to boil too long in the soap solution, as this will soften and eventually 

 destroy the ligaments uniting the elements. 



It is also well to have in the laboratory some disarticulated skeletons 

 in which the bones have been separated from each other and completely 

 cleaned by longer boiling in soap solution or in weak caustic soda. 



THE SKELETON 



In the frog, as in most Amphibia, the exoskeleton is practically 

 absent, and all parts may be regarded as forming the endo- 

 skeleton. 



This is composed of axial and appendicular portions. The 

 axial skeleton consists of the vertebrae, ribs, and skull; the appen- 



