CARCINOMA OF THE THYROID IN SALMONOID FISHES. 
377 
between the right and left halves of each pair of arches. The basibranchials are con- 
nected with each other by these same cartilaginous copulae, so that a solid mesal bridge 
is formed on the floor of the mouth, continuous with the tongue and reaching back to 
the fourth arch. The basibranchials lie opposite the interspaces between the arches, 
while the copulae lie opposite the arches themselves. The first and second basibranchials 
are well ossified, the third less so, and the fourth is mostly cartilage. 
Immediately beneath this bridge runs the ventral aorta. The bulbous aorta is 
located deeper, well beneath the floor of the mouth and somewhat back of the fourth 
arch. From it the ventral aorta runs dorsocephalad to a point just under the third 
arch. Here are given off the two trunks which soon bifurcate into the third and fourth 
branchial arteries supplying the corresponding gills. Thence the ventral aorta runs 
cephalad and slightly ventrad. Near the caudal margin of the second arch the second 
pair of branchial arteries is given off, supplying the second pair of gill arches. Finally 
the ventral aorta, just as it reaches the first arch, bifurcates into the first pair of branchial 
arches. A certain amount of space is left about the ventral aorta between it and the 
parts which inclose it. This space is greater dorsal to the vessel, and especially at the 
origins of the first and second pairs of branchial arteries. It contains the normal thyroid 
and its supporting tissues. 
Figures 15 and 16 illustrate the lateral, longitudinal, and dorsoventral distribution 
of normal thyroid. They represent the condition in no single fish, but show compositely 
a probable average from a number of individuals. Two chief masses of thyroid may 
usually be recognized, the follicles clustering at and back of the first and second pair of 
branchial arteries, with usually a definite space between them in which but few and 
scattered follicles occur. .Occasionally three masses or groups are recognizable, and the 
masses themselves present irregularities in the arrangement and number of follicles. 
Exact medisections show less thyroid than those slightly sagittal on account of the 
extension of the copulae and basibranchials to the aorta at the second and third arches 
on the mesal line. 
We have not found in the normal wild fish studied by us any lateral extension of thy- 
roid structure along the branchial arches. Most of it, in fact, hardly reaches the lateral 
margins of the mesal bridge. The most cephalic extensions rarely reach the first basi- 
branchial, and on the mesal line scarcely to the first copula. The extreme caudal exten- 
sion is to the fourth arch, but usually there are but few follicles either at or behind the 
third pair of branchial arteries. Follicles are present in bone spaces, but in normal wild 
trout we have never found them among the muscle bundles. They are frequently em- 
bedded in the fatty tissue network or lie loosely attached to the vessels or other tissues, 
but never show any invasive tendency, nor do the follicles occupy more than a part of 
the apparently available space of the thyroid region. 
ANOMALOUS DEPOSITS OF THYROID. 
In a quantitative sense all the thyroid of importance is confined to the immediate 
vicinity of a portion of the ventral aorta. As the thyroid is a somewhat diffuse organ 
