EXPERIMENTS UPON ANIMALS. 79 



tines after they have been exposed, by carefully opening the 

 abdomen. This is done in order to avoid the action of the 

 gastric juice. 



Celloidin sacs of small size are sometimes used to introduce 

 living cultures of bacteria into the bodies of animals without 

 their coming into direct contact with the tissues. 



Obtaining Material from Infected Animals. The animal 

 should be skinned, or the hairs plucked out, before it is washed, 

 at least the portion where the incision is to be made. Then the 

 entire body is washed in sublimate. Two sets of instruments 

 are required, one for coarser and one for finer work : the one 

 sterilized in the flame ; the other, to prevent being damaged, 

 heated in a hot air oven. 



The animal, the mouse for example, is stretched upon a board, 

 a nail or pin through each leg, and the head fixed with a pin 

 through the nose. The skin is dissected awaj r from the belly 

 without exposing the intestines. Then the ribs being laid bare, 

 the sternum is lifted up, and the pericardium exposed. A pla- 

 tinum needle dipped into the heart after the pericardium has 

 been slit will give sufficient material for starting a culture. If 

 the other organs are to be examined, further dissection is made. 

 If the intestines were first to be looked at, they would be laid 

 bare first. 



In this manner material is obtained, and the results of inocu- 

 lation noted. 



Frequent sterilization of the instruments is desirable. 



Koch's Rules in Regard to Bacterial Cause of Disease, 

 Before a microbe can be said to be the cause of a disease, it 

 must 



First. Be found in the tissue or secretions of the animal suf- 

 fering from, or dead with the disease. 



Second. It must be cultivated outside of the body on artificial 

 media. 



Third. A culture so obtained must produce the disease in 

 question when it is introduced into the body of a healthy 

 animal. 



Fourth. The same germ must then again be found in the 

 animal so inoculated. 



