THE ANATOMY OF THE THYROID GLAND 159 



presence of an increased amount of connective tissue between its 

 vesicles. 



On page 26 Guiard says that the thyroid gland "of fish" is always 

 unpaired; it is quite obvious that this remark should apply only to 

 the Elasmobraiichs, the only order of fishes which Guiard appears 

 to have studied. 



Bridge ('04) passes the thyroid gland with the brief statement 

 that '4n adult Elasmobranchs the thyroid is represented by a 

 moderately large compact organ, situated near the anterior end 

 of the ventral aorta." Although he describes the gland as one of 

 the ''blood glands" in connection with the vascular system, he 

 does not mention, nor indicate in any way, the source of its blood 

 supply. The statement of its intimate relation with the aortic 

 bifurcation might well lead one to erroneously suspect a supply 

 from this source. In quite another place (page 332) he speaks of 

 "a remarkable system of arteries for the supply of nutrient blood 

 to the gills and heart," which takes origin from the ventral ends 

 of the loops about the gill slits, the commissural vessels forming 

 by their union the "median longitudinal hypobranchial artery 

 which lies beneath the ventral aorta." He fails to mention the 

 ultimate ramifications of this system of vessels or its relation to 

 the thyroid gland, falling into the same error in this particular 

 as T. J. Parker, from whose plates Bridge takes his figures, and 

 upon whose description he appears to have largely based his 

 text. 



The literature upon the blood supply of the Elasmobranch 

 thyroid begins with Hyrtl ('58) who first described the hypo- 

 branchial arterial system in the Batoidei, if we except the very 

 incomplete description by Monro (1787). Hyrtl described the 

 thyroid artery as the "Ramus thyreoideus seu submentalis" 

 which takes origin from the 'Vein" of the second gill sac, and which 

 gives off muscular branches to that part of the oral mucosa which 

 lies between the inferior maxilla and the tongue bone as well as the 

 Glandula thyreoidea." Hyrtl did not at this time describe a 

 median hypobranchial artery, this vessel being represented in his 

 description by two anastomosing vessels on either side of the me- 

 dian line which arise from the second and third arches and which 



