No. 7. DEPART^MENT OF AGRICULTURE. 185 



quently the case, they are highly suggestive of the existence of rabies 

 and a laboratory examination must be resorted to. As the laboratory 

 examination is limited to the brain, nerve ganglion and upper por- 

 tion of the spinal cord, it is sufticient to submit to the laboratory 

 the dog's head and neck cut off close to the shoulders. In shooting 

 an animal through the head, the portion of the brain tissue usually 

 examined is apt to be destroyed and, therefore, it is recommended 

 that an animal be shot through the heart or destroyed with chloro- 

 form or strychnin. Of the large animals, horse, cow, hog, etc., the 

 brain may be removed and sent more conveniently and with less ex- 

 pense. Rapid decomposition is prevented when the specimen is 

 wrapped in paper or cloth and packed in a metal can or box sur- 

 rounded with ice in a shipping box. A metal or wooden bucket con- 

 taining the specimen wrapped in paper or cloth is considerably 

 used, and serves the purpose well. Glycerin has long been known to 

 have little effect on the virus of rabies and experimental animals may 

 be successfully inoculated with brain tissue immersed in glycerin for 

 months, and as the microscopic examination of the tissue is not much 

 interferred with, in glycerin immersed specimens, the specimens sent 

 to the laborator}^ in glycerin (water and glycerin equal parts fre- 

 quently used) are almost as desirable as the fresh specimens. 



ANIMAL INOCULATION TEST 



To reproduce rabies with material suspected of containing the 

 virus is absolute proof of its existence in the material. It is well 

 known that the saliva carries the infectious material or virus. Proof 

 of this lies in the manner in which the disease is transmitted, under 

 natural conditions, through the bite of an animal with rabies, the 

 saliva being the only material left behind as the bite is inflicted. In 

 the work of Pasteur, in the general study of the disease, he found that 

 of experimental animals inoculated with saliva containing the virus, 

 a large percentage not only failed to develop the disease, but also 

 that the period of incubation in those that developed symptoms and 

 died was very uncertain. Sound experimental work was therefore 

 impossible until a trustworthy method was found whereby the dis- 

 ease could be reproduced in a high percentage of all the experimental 

 animals inoculated with material containing the virus. Thus, Pas- 

 teur was led to use emulsions of spinal cord or brain tissue and it 

 was found that the virus in these structures was least liable to be 

 contaminated, more constant and probably in a more concentrated 

 form. The various ways of injecting the virus show that the seat of 

 injection influences not only the percentage of mortality, but also 

 the length of the period of incubation and, therefore, cutaneous in- 

 jections or infection through a wound in the skin, subcutaneous, in- 

 traperitoneal, intramuscular and intravenous injections were tried 

 and still the percentage of animals that would not develop rabies 

 was too large and the period of incubation too varied to consider 

 the results uniform. The best results Avere not obtained until the 

 distance from the point of injection to the spinal cord or brain tis- 

 sue, particularly the last named structure, was reduced to a mini- 

 mum, by injecting material containing the virus directly over the 

 brain or spinal cord, beneath the meninges — subdurally. As a means 



