CARCINOMA OF THE THYROID IN SALMONOID FISHES. 
383 
Fish which are without any clinical evidence of thyroid disease, either in externally 
visible swelling or the inflamed and flushed condition of the floor of the mouth, are for 
brevity referred to in this report as clean or clinically clean. 
It is the increased vascularity associated with increased thyroid that causes the 
red floor, and an inereased amount of hyperemic thyroid tissue of normal type sometimes 
makes a visible flush on the floor of the mouth so that the red floor is not an absolute 
and infallible sign of thyroid hyperplasia. The wild brook trout held in confinement 
in cement tanks and fed natural food showed after two years a number of cases of red 
floors. (See table viii.) Microscopically the thyroid was considerably increased in 
amount but not otherwise definitely changed from the normal type. Likewise the 
adult Scotch sea trout show occasional red floors referable to the microscopical condition 
of colloid goiter. Such cases of red floors are evidently to be separated from those in 
which the flush is caused by the early stage of carcinoma. 
visible tumors. 
Branchial junction . — Whereas evidence of beginning growth of thyroid tissue in 
the affected fish is first shown by a more or less distinct reddening of the floor of the 
mouth, the growth of tumor tissue in the spaces about the aorta and in the muscular 
structure of the isthmus may proceed to a very considerable extent before the growth 
gives other macroscopic evidence of its presence. The illustrations of visible tumors 
in the literature usually depict growths appearing at the branchial junction. Because 
of the resistance offered to the growth of the tumor by the bony and cartilaginous struc- 
tures forming the floor of the mouth, it is natural to expect that the region at the junction 
of the first pairs of gills with the isthmus, representing the line of least resistance, would 
be the first and most frequent site of the visible outgrowths. This median region we 
refer to as the branchial junction. 
In an analysis of 91 tumors carefully classified (table i), we find that the branchial 
junction alone is the site of the visible tumors in but 4 cases, but where the growth of 
tumor tissue is sufficiently extended to protrude in other directions it is found to be the 
site of visible outgrowths in combination with one or the other, in 67 cases. The branch- 
ial junction is therefore one of the most common sites for the early evidence of tumor 
growth. In many cases the first macroscopic evidence of the disease is found in the 
appearance of small protrusions with smooth surface, of rose color, often not larger 
than a grain of rice or smaller, on each side of the isthmus exactly at the branchial 
junction. This evidence of tumor formation is almost always associated with the 
evidences of reddening of the floor of the mouth and means that the avaliable space 
below the branchial arches and about the aorta is filled with tumor mass and that the 
conditions favoring the growth downward of the tumor have determined its first pro- 
trusion in this direction. Tumors making their first appearance at the branchial junction 
on one or either side of the isthmus may rapidly develop into large growths in this region. 
As they increase in size they become more readily recognizable, push the gill covers 
apart and ultimately present themselves as obvious growths protruding into the gill 
spaces on either side. 
