THE ANATOMY OF THE THYROID GLAND 181 



its anterior border, distributing its main branches to the substance 

 of the gland and small collateral branches to the floor of the 

 pharynx in front of the hyoid arch and to the anterior third of the 

 coraco-hyoideus and coraco-mandibularis muscles. A small median 

 unpaired vessel arising from the left thyroid artery (less frequently 

 from the right), penetrates the thyroid gland, divides, and enters 

 the coraco-hyoideus to supply the antero-median portion of this 

 muscle. This is very probably homologous with the anterior 

 portion of the arteria thyroidea impar, derived from the median 

 hypobranchial as described by Hyrtl (72). 



The left thyroid artery is usually larger, longer, and more 

 extensive as to its area of distribution than the right. Lombard 

 ('09) dissected a number of specimens of Mustelus and Raia and 

 found that the left thyroid artery more frequently entered the 

 dorsal surface, and the right the ventral surface, of the thyroid 

 gland. I have found a somewhat similar condition, though I very 

 frequently find both vessels coursing upon the ventral surface 

 and sending their branches dorsally into the substance of the gland. 

 Occasionally the right thyroid artery enters the dorsal surface of 

 the gland and the left the ventral (fig. 3 and 19). The position and 

 distribution of the thyroid arteries is, however, subject to consider- 

 able variation and, as I have already pointed out, the right may 

 even supplj^ a greater portion of the gland than the left thyroid 

 artery (figs. 3 and 4). 



The thyroid artery as described very probably in part corre- 

 sponds to the vessel which was recognized by T. J. Parker ('86) as 

 the coraco-mandibular artery. This latter is an obviously inaccu- 

 rate designation, for the coraco-mandibular branches are insignifi- 

 cant as compared with the other ramifications of the artery. 

 Hyrtl ('72) recognized and more accurately described the thyroid 

 artery as arising from the "veins of the first gill-arch" in conjunc- 

 tion with the submental artery; his description appears to be 

 accurate with the exception that the two arteries in my dissec- 

 tions appear more frequently to arise independently. 



The observations which I have recorded concerning the origin 



THE AMERICAN JOURNAL OP ANATOMY. VOL. 11, No. 2 



