384 
BULLETIN OF THE BUREAU OF FISHERIES. 
In the diagrams of the 48 analyzed tumors here presented, great irregularity of out- 
line, size and form of these outgrowths on the inferior aspect of the fish may be easily 
studied. The freedom afforded by the spaces between the gill covers and the isthmus 
permits of the growths of tumors of enormous size. Figure 27 represents a 2 -year-old 
brook trout with a tumor of such magnitude that practically the entire gill space is 
filled by one tumor mass, which has pushed the isthmus downward and practically oc- 
cludes the entire gill space. In some cases tumors of great size may develop in such a 
way as to push the isthmus to one side and present the appearance of a unilateral tumor. 
In all these cases, however, the tumor tissue is found to extend from the median region 
about the aorta, which is the site of the normal thyroid tissue from which all these 
tumors spring. Occasionally tumors extend downward and make their first appearance 
through the substance of the isthmus, indicating the exquisite infiltrative character 
of the growth, and in one instance, the first macroscopic evidence of tumor (fig. 4) was 
found at the base of the isthmus in the solid muscular structure of the breast of the fish. 
Floor of the mouth . — Although the lines of least resistance for the growth of tumor 
tissue springing from the thyroid region are obviously downward, a large proportion 
of all tumors give evidence of their existence in the floor of the mouth. In an analysis 
of 91 fish taken at random, visible evidence of tumor growth in the floor of the mouth 
was found in 70 instances. The reddening of the floor of the mouth, which is the first 
evidence of tumor growth, is replaced by infiltration through the structures of the floor 
of the mouth, developing either by uniform bulging upward of the floor or the protrusions 
of masses between the branchial junctions; or one or more rounded masses of large dimen- 
sion may become prominent. (Fig. 26 and 29.) Not infrequently, besides the upheaval 
of the floor, the tumor tissue breaks through the elastic structure of the underlying 
mucosa and produces minute papillary growths. These are usually of shiny appear- 
ance, small or rounded, irregularly shaped, and present the appearance of vegetations 
scattered over the floor of the mouth. (Fig. 30.) 
Pit tumors . — Early in our investigations Marsh called attention to the fact that the 
jugular pit often contained a tumor which was entirely independent of the thyroid 
enlargement originating in the thyroid region. (Fig. 24.) Of the 91 tumors analyzed, 
in 5 the only macroscopic evidence of tumor growth was in the jugular pit. In combina- 
tion with other regions the jugular pit was the site of tumor growth in 20 other examples, 
and in the entire 91 it was the site of tumor growth in 25. As has been shown in the 
chapter on embryology, we were able to demonstrate the existence of deposits of normal 
thyroid tissue in the immediate vicinity of this so-called jugular pit. The frequency 
of tumors in this region strongly indicates that deposits of thyroid in this region are not 
uncommon, and in determining the limitation of distribution of the thyroid in normal 
fish, the frequency of displacement of occasional follicles in this region must be recog- 
nized. As will be seen from the diagrams, the jugular pit may be the site of outgrowth 
of tumor tissue of very considerable extent, and of various forms. They are usually 
subspherical, or hemispherical, may be slightly flattened or considerably elongated, 
and occasionally vegetate irregularly and take on bizarre forms. They vary greatly in 
size, from bare visibility to masses having a diameter of 17 millimeters or even more. 
