The Attenuation Method 



387 



series until a standard virulence is attained and the virus is said to be "fixed." 

 It has a much higher degree of virulence than the "street virus" taken from the 

 rabid dog, but its virulence does not vary. In most laboratories the "fixed 

 virus" is obtained from other laboratories and kept passing through rabbits. 

 In this manner uniformity of dosage and virulence is most easily maintained. 

 The technic of obtaining the rabbit's cord given by Oshida* is the one now 

 generally employed. As given by Stimson,t it is performed at the Hygienic 

 Laboratory as follows: "The rabbit, when completely paralyzed, is killed with 

 chloroform and nailed to a board, back uppermost, and thoroughly wetted 

 down with an aseptic solution (i per cent, trikresol) . An incision is made through 

 the skin from the forehead nearly to the tail and the skin laid back on each 

 side, the ears being cut close to the head. An areai inch wide is seared with 

 a hot iron around the occiput and nuchal region and ear openings. The skull 



Fig. 134. — Removal of the spinal cord from a rabbit (Stimson, Bull. No. 65, 

 Hygienic Laboratory). 



is then transversely divided in the center of the seared areas by means of bone- 

 cuttmg forceps. The neck is dissected loose from the skin and a large square of 

 sterile gauze is inserted beneath it. The lumbar region is dissected up for a few 

 mches and a similar piece of gauze placed beneath it. Then a piece of telegraph 

 wire about 14 inches long, bent into a handle at one end and having a small 

 wisp of cotton twisted about the other end, is used to push the cord out of its 

 canal. The spine is steadied by a pair of lion-jawed forceps. 



An assistant catches the cord with forceps as it emerges from the cervical 

 openmg and lifts it out. The spinal nerves are torn off during this procedure, 

 and the membranes stripped off, leaving a clean sterile cord. A silk ligature with 

 one long end is placed around the upper end, and another, just below the middle 

 of the cord, whiph is then cut into two pieces just above the lower ligature. A 

 small piece is cut off of the upper end of the upper portion and placed in a tube 

 of bouillon, which is incubated as a test for sterility. The cords are hung in the 

 drying bottle over sticks of caustic potash or calcium chloride. 



The longer the cord dries, the more the virulence of the micro- 

 ■ organisms attenuates. 



When the cord has reached the necessary attenuation, i cm. of 



*"Centralbl. f. Bakt. u. Parasitenk.," 1901, xxdc, Orig., 988. 

 t "Facts and Problems of Rabies," Hygienic Laboratory Bulletin No. 65, 

 June, 1910, Washington, D. C. 



