490 INFECTION AND RESISTANCE 



ordinary street virus as obtained from rabid animals passed through 

 a considerable series of rabbits (20-30) until its virulence for these 

 animals has reached a maximum. After a sufficient number of such 

 rabbit passages the incubation time after intracerebral inoculation is 

 reduced to 7 or 8 days, but can no longer be shortened by further 

 passage. The brain and cord material of rabbits dead of rabies after 

 such repeated passages constitutes virus fixe. This can be preserved 

 for considerable periods in 60 per cent, glycerin, and this is the 

 initial material from which the attenuated preparations for treat- 

 ment are produced. 



In preparing the material for treatment a small amount of virus 

 fixe is injected subdurally into rabbits, about 0.2 c. c. of a salt solu- 

 tion emulsion being given. The inoculation is very easily made 

 through a small trephine opening in the skull, and contamination 

 is very easily avoided. Just before the rabbit dies when completely 

 paralyzed it is killed by chloroform and the cord is removed best by 

 the method of Oschida. 100 The rabbit is nailed to a board, back up- 

 permost, and washed with a weak antiseptic, a longitudinal incision 

 is then made along the backbone from the occiput to the lumbar 

 region, and the vertebral column laid bare. 



After searing the tissues around the back of the head the spine is 

 cut across just behind the occiput, and again in the same way just 

 above the sacrum. The neck and lumbar regions are dissected loose 

 from the skin and gauze is inserted under it to avoid contamination. 

 The assistant grasps the end of the spinal cord as it appears in the 

 cervical region and pulls on it very slightly while the operator with 

 a glass rod or a piece of wire pushes against it from below. If this 

 is carefully done the spinal nerves are torn and the cord can be 

 gradually pulled out of place. This procedure is by far the best, 

 although it requires a certain amount of practice. 



The cords so removed are hung up by a thread in bottles contain- 

 ing sticks of caustic potash and exposed in a dark place to 22 to 23 

 C. Under these conditions of drying and temperature the virus is 

 gradually attenuated until at the end of 13 days or more the viru- 

 lence is practically nil. If removed from the drying bottles at any 

 time during the process and kept in a refrigerator in sterile glycerin 

 the virulence, whatever it may be at the time of placing into the 

 glycerin, remains fairly constant for long periods. When any of 

 this material is used for treatment little pieces of the cord % cm - i n 

 length are cut off and emulsified in 2.5 c. c. of salt solution, and this 

 emulsion is used for injection. 101 



100 Oschida. CentraXbl. f. Bakt., Vol. 29, 1901. 



101 In our description of the methods of drying 1 rabies, for the sake of 

 adhering to a standard, we follow closely the directions laid down by A. M. 

 Stimson, in the U. S. P. H. S. Bull. 65, 1910. There are various modifica- 

 tions used in different countries, in many cases unimportant, and it seems 

 well to adhere to the U. S. regulations as a standard for this country. 



