378 Fortieth Annual Meeting 



under the junction of the pair of second gill arches. 

 Through this region beneath the floor of the mouth on the 

 median line passes the ventral aorta, giving off branches 

 to the gills. The particles making up the gland are dis- 

 tributed immediately about this vessel, and are scattered to 

 some extent about the adjacent region, even out along the 

 gill arches on either side. The thyroid is not recognizable 

 to the naked eye as a distinct and definite gland or organ 

 because of its small size and the separation of its units and 

 their distribution among other tissues. By an ordinary 

 dissection of the fish the gland would scarcely be discovered. 

 Even in microscopic sections it is an obscure and easily 

 overlooked tissue. Yet under an unknown stimulus it may 

 grow until it appears externally and becomes larger than 

 any other organ of the body. 



The first external indication of thyroid enlargement is a 

 red streak or spot on the floor of the mouth at or near 

 the second pair of arches, the so-called "red floor." It 

 usually, probably always, precedes any externally visible 

 enlargement when the thyroid is growing upward. This 

 reddening area on the floor of the mouth indicates the in- 

 creasing blood supply accompanying the increase of thyroid 

 tissue. The direction of the thyroid growth may be entirely 

 downward, doubtless without the tell-tale symptom of the 

 red floor. The continuance of growth may be in almost 

 any direction. The lines of least resistance are sidewise 

 and downward, and here the tumor is most often brought 

 externally visible, though it frequently penetrates the floor 

 of the mouth, which is chiefly cartilaginous, and occupies 

 space directly within the mouth cavity. As it swells out- 

 ward it carries with it the thin skin, or epithelial covering 

 about the throat, and this skin becomes the covering of the 

 tumor. In advanced cases the tumor infiltrates or grows 

 into the skin as well as extending it by pressure. The 

 small terminal gill filaments of the foremost gill arches 

 extend out upon this epithelium, and are often seen stretched 

 by large tumors till they can carry no blood. Almost every 



