OBSERVATION OF INFECTED ANIMALS. 



273 



numbered and designated by their color, by their curly or 

 smooth hair, etc., and, if need be, by painting the nose, 

 back or flank with anilin dyes. The student should never 

 burden his memory with details as to dose, time of inocula- 

 tion, etc. A complete record should be made at the time 

 the work is done, and in this way all uncertainty is done 

 away with. 



The experimental animals, as a rule, should be weighed 

 before infection, and every day or two afterward. Each 

 weighing should be done, as nearly as possible, under the 

 same conditions as the first one. 

 That is to say, the animal should 

 have plenty of jood and water before 

 weighing. In warm weather the need 

 of water is especially imperative. 

 An experimental animal may show 

 a difference of 200 or 300 g. between 

 the two weighings, and this differ- 

 ence may be wholly due to the with- 

 holding of water. Unless the precau- 

 tions mentioned are observed the 

 weighing results will have no value 

 whatsoever. When properly carried 

 out, it affords the best possible index 



of the physical well-being of the animal. A steady, though 

 slight decrease in weight, is a sure indication of a chronic, 

 wasting disease. On the other hand, a steady increase in 

 weight is, as a rule, a reliable index of health. 



The temperature of the animal should be taken before 

 beginning the experiment, and subsequently on each suc- 

 cessive day at the same hour. The ordinary clinical ther- 

 mometer should be inserted well up into the rectum. Pre- 

 vious to insertion, the bulb should be covered with vaselin. 

 The variations in temperature are especially important 

 when studying the action of bacterial poisons on the animal 

 body. 



FIG. 46. a Rat cage with 



18 



