THE FROG. 



Each student should have an injected frog and a tadpole for 

 dissection, a decalcified head for the study of the brain, and 

 a large tadpole. If possible there should also be living frogs 

 for study. The injection should be made through the ventricle 

 which will fill the whole arterial system. Decalcified heads 

 should be prepared by placing them in nitric alcohol about a 

 week before they are to be used, and then washing them in 

 running water for at least two hours. 



Skeletons are easily prepared * and there should be one for 

 every three or four students. 



In the living frog notice the way in which the eyes 

 can be retracted. Notice especially the way in which a 

 frog breathes, observing the skin underneath the throat, 

 and watching the nostrils through a hand-lens. On the 

 back, a little in front of the vent, pulsations may be seen ; 

 these are produced by a pair of lymph-hearts beneath the 

 skin. 



* A skeleton may be quickly prepared by removing as much 

 of the flesh as possible with scissors and scalpel, then boiling it 

 with a little soap in the water, and picking away as much more 

 flesh as you can, taking care not to separate the joints. Much 

 better skeletons can be made by cleaning off the flesh and then 

 soaking the frog for some weeks in water, brushing the parts every 

 few days with a tooth-brush. If such a skeleton be soaked for a 

 few days in a weak solution of glycerine and dried, it will retain 

 its flexibility and usefulness for years. 



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