BY T. HARVEY JOHNSTON AND M. J. BANCROFT. 287 



exhibited by the inner surface of the skin of an animal 

 which has been dipped in a sohition only slighly too strong, 

 would lead us to suppose that the amount of arsenic present 

 in the blood circulating in the most peripheral vessels- 



of the skin would be verj^ considerable It is 



quite conceivable that . . . this peripheral blood con- 

 taining a large quantity of arsenic would be immediately 

 diluted by the general mass of blood from the internal parts 

 of the animal so that the total amount in general circulation 

 would not be excessive." 



In experiments where arsenic was injected subcutane- 

 ously, only those ticks in the immediate vicinity of the 

 puncture were killed, those a few inches away being 

 unaffected (Cooper and Laws, p. 208-9). These authors, 

 along with Watkins-Pitchford. believe that " arsenic is 

 cumulative in its action and that the quantity absorbed by 

 the tissues of the skin is augmented by each subsequent 

 dipping until a certain maximum is reached/' the excess 

 being absorbed into the blood stream and eliminated into 

 the urine b}' the kidneys (p. 200). Analyses of the skin, 

 of animals dipped at short intervals has proved that 

 arsenic does accumulate as a result of repeated application, 

 penetration of the poison taking place as a result of osmosis 

 (p. 201). 



If Mr. Hull's observation, that dipping suppresses 

 resistance, be correct then one might explain it by assuming 

 that the arsenic absorbed as a result of one or of a few 

 dippings, is sufficient to alter the anti-tick quality of the 

 blood or lymph and thus convert the resistant cow into an 

 ordinary cow so far as tick infestation is concerned. Since 

 the protective effect of arsenic lasts only for a few days 

 after dipping,* one would have to assume that the influence 

 of the arsenic as a tick destroyer does not persist as long as 

 its influence in suppressing the hypothetical anti-tick quality 

 in peripheral blood. 



We are not satisfied that arsenical dipping by itself 

 will suppress tick resistance since loss of condition would 

 probably afford a sufficient explanation in the cases 

 mentioned by Mr. Hull, 



*GraybiU. U.S.D.A., B.A.I., Bull. 167, 1913; Watkins-Pitchford 

 in Cooper and Laws, I.e., p. 201. 



