THE DUCTLESS GLANDS 321 
bony skeleton in the form of chondrodystrophy or of 
rickets, while some degree of osseous change is observed 
Avith myxedema. Goitre, be it simple or exophthalmic, may 
be followed by cardiac enlargement or by myocarditis. 
The character of the thyroid secretion is not kno^vn 
except that it is influenced by the availability of iodine in 
the diet, but there are some other as yet unexplained 
features. The normal thyroid fed to cretins or persons 
suffering with myxedema, has the power to improve the 
condition apparently by supplying iodine and the other 
essential elements. Iodine is an important constituent 
of the gland, being present in combination with protein. 
Its quality varies indirectly with the amount of colloid 
and of hyperplasia (Marine). The administration of this 
element is beneficial in colloid goitre but is harmful in the 
toxic variety. Even though the administration of thyroid 
extract may relieve athyroidism, this procedure in normal 
animals fails to produce typical pictures of hyperthyroid- 
ism. Carnivora fed thyroid gland do not show toxic 
symptoms until excessive amounts are given, whereas 
herbivorous varieties are much more sensitive to this 
feeding. (4) Tachycardia, nervousness and exophthalmos 
are not produced by these experiments, an interesting 
observation since these signs are not recorded in wild 
animals, and only vaguely reported by veterinarians. 
Man is apparently very sensitive to thyroid dysfunction. 
On the other hand, meat-eating animals are more 
sensitive to excision of the thyroid body than are grain 
eaters. (5) Chemical studies have shown that the thyroid 
is concerned in basal metabolism since this is increased 
in hyperthyroidism and decreased in myxedema; nitro- 
gen output is much elevated. 
Thyroid physiology stands probably in some relation 
to the cardiac mechanism since in simple colloid or simple 
hyperplastic goitre if of long duration, cardiac enlarge- 
(4) Carlson, Rooks and McKie, Loc. cit. 
(5) Vincent and Jolly, Loc. cit. 
