THE DUCTLESS GLANDS 317 
quadrupeds it may become dependent and the swelling is 
nearer the sternum than is the case in man. The isthmus 
is a very variable structure, and its presence or absence 
cannot be said to be a constant character in any order, or 
indeed in any family, I have seen in old animals a fibrous 
band extending over the face of the trachea connecting 
the capsule of the lateral lobes, which might have been 
an isthmus at one time. From these few observations the 
idea of atrophic fibrosis occurred to me. Such may be 
the reason for the absence of this transverse link in some 
adult specimens. 
The amount of thyroid tissue possessed by an animal 
might be judged by measurement or weight. The former 
is misleading since the density might vary, as it certainly 
does in the two classes and between certain orders in 
mammals. Actual weights would afford little comparison, 
whereas the weight in terms of total body weight may 
supply a guide to the amount of gland normal to an 
animal. There are given in Table 15 the grams-per-kilo- 
gram-body-weights of the thyroid bodies (thyroid and 
parathyroids both sides combined) of twenty animals 
whose gland seemed entirely normal at autopsy. They 
are all adult specimens, free of cretinoid characters and 
of bone or heart diseases, conditions which might reflect 
abnormalities to these glands. The list is too small to 
warrant any conclusions, but in one respect confirms 
Murray's (1) observation and certain experimental work, 
notably of Vincent and Jolly (2) and Carlson, Rooks and 
McKie (3). The carnivores have more thyroid than 
ungulates (averages .55 gm. vs. .18 gm,), but the mar- 
supials on our list have nearly as high an average as the 
former, namely .44 gm. To these figures might be added 
others which I have worked out from the list given by 
Murray ; it is only possible to compute the gram-per- 
(1) Proc. L. Z. Boc, 1919, p. 16. 
(2) Journ. Phys., Vol. 34, 295. 
(3) Am. Jour. Phys., Vol. 30, 129. 
