51 
doses an increase of 10 per cent in the amount may produce twice 
the effect. With still other doses the amount administered may be 
increased many times before twice the effect is produced. 
The relation between the percentage of iodine in thyroid and its 
activity in increasing the resistance of mice to acetonitrile seems to 
offer an unusually favorable opportunity for a study of this char- 
acter, but we have not been able to devote special attention to it. 
A brief discussion of the subject is, however, indispensable for an 
understanding of our results and the conclusions we draw from 
them. From the experiments already described (Series II, children’s 
thyroids, p. 35) it is evident that even a very weak thyroid prepara- 
tion will, when given in a sufficient dose, produce as great a physio- 
logical effect as an equal amount of a very active preparation. In 
other words, there is a limit to the degree of resistance which the 
body can acquire even under the most favorable circumstances. It 
is obviously necessary, in comparing the physiological effects of dif- 
ferent thyroid preparations, to avoid doses which produce a maximum 
effect. Also, as the doses approach the maximum, each successive 
increase produces a relatively smaller effect. A curve expressing 
these facts graphically is a parabola; at a certain point a prepara- 
tion with twice the activity of another will produce twice the effect; 
one with three times the activity would produce three times the 
effect, etc. The farther removed the doses are from this optimum 
the less close is the correspondence between the size of the dose of 
the preparation and its (measurable) physiological effect. The 
range through which there is a direct relation between the physio- 
logical activity of the thyroid and the amount of acetonitrile which 
may be rendered harmless under its influence seems to be rather 
great; this doubtlessly accounts for the comparative ease with which 
we have obtained quantitative results in the following experiments. 
In work of this character it is not only permissible but necessary 
to exclude experiments in which the thyroid produced maximum 
effects. Thus it would obviously be unwarranted to conclude from 
the experiments of Series II (p. 35) that the thyroid of children is as 
active as that of sheep because 0.05 gm. of the former protected 
against as much acetonitrile as did 0.05 gm. of the latter; in each 
case the amount fed was probably far above that necessary to pro- 
duce the maximum effect. 
The plan of our experiments was as follows: When the activity of 
several thyroid preparations was to be compared, they were fed in 
equal amounts, but care was taken that the quantities administered 
were such as we believed would produce submaximum effects. It 
was sometimes necessary to perform two or three series of experi- 
ments of this character before the amounts most suitable for com- 
parison could be determined; this was partly due to the fact that 
