90 
Conclusions from, the experiments with morphine. 
The resistance of rats, mice, and guinea pigs to morphine is uni- 
formly lowered by the feeding of thyroid. This occurs independently 
of any effect the thyroid may have upon the weight of the animals. 
In the case of the rat there is a close parallelism between the physio- 
logical effect of the thyroid, as determined by the increased suscepti- 
bility to morphine, and the percentage of iodine. A similar parallel- 
ism was found, in general, in the experiments on mice; the animals 
which had received thyroid with a higher percentage of iodine almost 
invariably died from a smaller dose of morphine than did those which 
had received thyroid with a low percentage of iodine. The question 
of the relation between the iodine content and the physiological 
activity of the thyroid was not tested on guinea pigs. The results 
of the experiments with morphine on rats and mice are therefore an 
additional argument for the view that the degree of physiological 
activity of thyroid is parallel to the iodine content. 
We have no explanation to offer for the effect of thyroid feeding 
upon the resistance of animals to morphine further than the sugges- 
tion made by one of us in a previous paper : a 
I have not been able to determine the cause of this increased susceptibility; 
possibly the thyroid, which is known to hasten the oxidation of fats, affects the lipoids 
of the nervous system in such a manner that poisons, such as morphine, gain access 
to it more easily. 
Mansf eld b found that rabbits which had been deprived of food 
for a few days were much more susceptible to morphine than were nor- 
mal rabbits; he endeavored to explain this in the terms of the Meyer- 
Overton theory of narcosis: In starvation there is a diminution of fat 
in the body, except in the central nervous system; as a result of this a 
larger proportion of the fat soluble narcotic is taken up by the nervous 
system, and so is able to exert a greater toxic action. Although 
thyroid has a tendency to cause a diminution of fat in the body, we 
do not believe that this can have played a large part in our experi- 
ments; in some of these the thyroid had increased the susceptibility 
of the animals to morphine ten times without causing any change in 
weight. 
These experiments may have some bearing upon the use of mor- 
phine in exophthalmic goiter (hyperthyroidism). There seems to be 
unanimity of opinion among clinicians that general anaesthetics are 
very dangerous in this condition. 0 The opinions differ as to the use 
a R. Hunt.. J. Am. M. Ass.. Chicago, 1907, 49 , p. 1325. 
b G. Mansfeld, Arch, intemat. de pharmacodynamie et de therapie, 1905, 15 , p. 467. 
C cf. Th. Kocher, Verhandl. d. deutsch. Gesellsch. f. Chir., Berl., 1901, 30te Kong., 
p. 346; A. Kocher, Mitt. a. d. Grenzgeb. d. Med. u. Chir., Jena, 1902, 9 , p. 1. Angerer 
(Munch, m. Wchschr., 1896, 43, p. 71) states that the feeding of thyroid to patients 
with goiter renders them especially liable to heart failure when a general anaesthetic 
is administered. 
