THE ANATOMY OF THE THYROID GLAND 183 



and course of the thyroid artery and the hypobranchial system 

 were carefully worked out with specimens of Mustelus and Squa- 

 lus and verified in all thetr important particulars in Carcharias 

 and Raia. 



VEINS AND LYMPHATICS OF THE THYROID REGION 



The numerous small veins of the thyroid gland discharge into 

 the ''thyroid sinus," which connects together the hyoid sinuses of 

 the two sides. The conformation of the hyoid sinuses and their 

 tributaries and connections have been well described by T. J. 

 Parker ('86). In addition to the transverse anastomosis formed 

 by the thyroid sinus, the hyoid sinus receives a submental vein 

 from the region of the mandible and numerous small muscular 

 branches from the neighboring muscles. This sinus and its con- 

 necting vessels can be most readily observed in Raia. The sub- 

 mental vein is seen to begin as a double transverse anastomosis; 

 the larger, anterior, tributary lies close behind the cartilage of the 

 inferior mandible; the smaller, posterior vessel arches across the 

 floor of the mouth just in front of the hyoid arch and the anterior 

 border of the thyroid gland. At the angle of the jaw these vessels 

 unite in a small sinus which also receives a transverse anastomo- 

 sis from in front of the maxilla, so that the mouth is thus encircled 

 by an annular venous sinus. The thyroid sinus similarly forms a 

 double transverse anastomosis, rather more deeply placed, behind 

 the hyoid arch at the posterior border of the gland. These vessels 

 convey the blood from the ventral cervical region to the hyoid 

 sinus. 



The thyroid veins open into the thyroid sinus as several small 

 branches, the largest of which are a median vein, leaving the organ 

 near the middle of its dorsal surface, and two anterior veins which 

 leave the same surface near the anterior margin of the organ, 

 but a little to either side of the median line. Other smaller veins 

 leave the lateral margins of the organ passing either to the thyroid 

 or the hyoid sinus. The thyroid veins must contain valves, for 

 although the vessels can be readily traced with the dissecting 

 microscope and even with the naked eye, it is with difficulty that 



