THE ANATOMY OF THE THYROID GLAND 193 



The very large relative size of the follicles of Raia is at once 

 apparent. They are approximately four times as large, relatively 

 to the length of the fish, as in the case of any other species. When 

 compared with the diameter of the gland the ratio is again in- 

 creased, but this difference is in part compensated for by the 

 increased thickness of the gland in Raia as compared with the 

 other species. The thyroid gland of Raia is 1,5 times as thick as 

 that of Carcharias, and 2 to 2,5 times the thickness of the gland 

 in Mustelus. 



The column of ratios in the above table would indicate that in 

 the Selachians the size of the follicle is in approximate proportion 

 to the size of the fish, but that in Raia the relative size of the 

 follicle is many times as great; the actual number of follicles in the 

 thyroid gland of Raia is only a small fraction of those in the gland 

 of any of the other species. It is readily susceptible of mathemat- 

 ical proof that the combined circumference of large follicles con- 

 tained in a given area is less than the combined circumference of 

 smaller follicles in the same area; hence the gland cf Raia with its 

 larger follicles will contain proportionately less epithelium than 

 the glands of the other species. I estimate that the difference 

 is just about sufficient to render the volume of secretory epithelium 

 in the gland of Raia relative to the size of the fish equal to the 

 volume of secretory epithelium in each of the other species. 



But it is equally susceptible of mathematical proof that the 

 cubical contents of the combined follicles is greater in the 

 gland having the larger folhcles; hence there is in Raia a greater 

 volume of intrafollicular space than in the other species. It is 

 scarcely susceptible of proof but entirely reasonable to suppose 

 that the epithelium of different individuals of the same or different 

 species so closely allied and the Batoidei and the Selachii is approx- 

 imately equally active as regards its secretory function. There is 

 ample evidence that the fluid secreted by the thyroid epithelium 

 into the cavity of the follicle finds its way through the wall of the 

 follicle to the neighboring vascular spaces so that the direction of 

 flow must, in part at least, be from the epithelium into the follicle 

 and thence through the follicular wall to the vessels. In view of 

 these facts the rate of this secretory flow in Raia, with its relatively 



