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 tlie 

 red floor, and an increased 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 available 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. 



