J. D. E. Holmes 
81 
their flagellated form, but those destined to continue the species reform 
into a latent or cystic body which comes to rest in some internal organs, 
probably the liver or bone marrow, during the period of intermission, 
finally developing into flagellate forms, which again make their way into 
the blood streams. 
The mature trypanosome is easily destroyed by arsenic, antimony and 
other drugs. The action of arsenic is direct and takes place when it is 
being carried through the tissues in the circulation. Arsenic acts but 
for a brief period, as it is rapidly eliminated from the circulation. The 
rapidity of the destructive action of arsenic on the trypanosomes is in 
direct proportion to the dose. Consequently, it is possible, if the dose 
is sufficiently large, to sterilise the tissues before the arsenic is removed. 
With smaller doses numbers of the parasites are not destroyed and, 
entering into a latent state, remain to cause a relapse. This hypothesis 
of the life stages of the trypanosomes would also explain the successful 
results of the treatment with arsenic given in full doses at intervals and 
the non-success of the continuous dosage with smaller amount. The 
toxic effect of arsenic on the system is cumulative, but the action on the 
trypanosomes is apparently not so. Arsenic in the circulation has, 
probably, little or no effect on the latent bodies, and the accumulated 
amount of the drug is stored up in the liver and other organs, where it 
is not brought in contact with the mature parasites. 
I have to acknowledge the co-operation of Mr Cross, Assistant 
Bacteriologist, in these experiments, and the assistance of Dr Hartley, 
Physiological Chemist, in questions connected with chemistry. 
The Bulletin of the Bureau of Sleeping Sickness has been extensively 
used for purposes of reference to other works on trypanosomiases. 
Toxic Dose of Arsenious Acid, Soamin and Atoxyl. 
Arsenious acid. 
The toxic dose for the horse is said to be (Law) 140 grains in solution 
or 14 grains in solid form; (Kaufmann) 3‘4 grammes in solution or 
45 grammes in solid form ; Trelut administered 3 to 6 grammes daily in 
treatment for dourine. 
In our experiments it was found that ponies of about 300 lbs. could 
tolerate repeated doses of 1 gramme without ill effect; that a dose of 2 to 3 
grammes was fatal for ponies of this weight; that horses weighing from 
900 to 1000 lbs. could not tolerate more that 3 grammes. 
Parasitology in 
6 
