200 
Theory and Practice of Dipping 
action of the arsenic did not range beyond a margin of six inches sur¬ 
rounding these areas (see Appendix V). 
The general conclusion to be drawn from all these experiments is 
that the tick does not obtain a lethal dose from arsenic present in the 
circulating blood. 
Cumulative Action of Arsenic in Dipping. 
Although all the ticks present on an animal at the time of dipping 
may be killed by a single immersion, yet this does not prevent reinfesta¬ 
tion by ticks subsequently picked up in the grazing area. To prevent 
reinfestation it is necessary to repeat the dipping at short intervals and 
in order to do this without injury to the stock, the concentration of 
the dipping bath must be reduced accordingly. It is then found that, 
at the reduced strength, although a single dipping fails to kill all the 
attached ticks, repeated dippings render the host poisonous to all 
ticks, both those attached to the host at the time of dipping and also 
those ticks which may be picked up later. Now, a single immersion is 
ineffective, therefore, the only conclusion to be drawn is that the 
arsenic is cumulative in its action and that the quantity absorbed by 
the tissues of the skin is augmented by each subsequent dipping until, 
as will be shown later, a certain maximum is reached. 
From Watkins-Pitchford’s statements, it is obvious that he holds 
similar views to ourselves with regard to this cumulative effect of 
short interval dipping, and the affinity of the skin of an habituated 
animal for arsenic. Concerning this he writes (191 lu, pp. 47-48) as 
follows: 
‘In fact, as will be seen from the succeeding observations, the 
deeper layers of the skin appear to be capable of becoming so tolerant 
of the presence of arsenic that the latter becomes tolerated to a con¬ 
siderable extent. That this accumulation is not a mere mechanical 
deposition or passive soaking is indicated in Schedule E, items 5 
and 5a. This indication of the vital action of the skin is further 
borne out by the fact that any arsenic in excess of the maximum 
content is eliminated from the skin, the elimination taking place 
through absorption by the blood-vessels which are contained in 
its deeper layers, such excess of arsenic appearing shortly afterwards 
in the urine. When we further consider it is into this deep layer 
of*the skin that the tick thrusts its mouth parts and obtains its 
nourishment, we shall partly see the significance of being able to 
establish and maintain a supply of arsenic at such a point of attack.’ 
