THE THYROID GLAND AND ITS DISEASES 207 



The variability of the thyroid gland is probably due to several factors. 

 While it usually develops from a single median ventral outpouching of 

 the pharyngeal epithelium, the contributions from the fifth branchial 

 clefts, common in lower forms, may persist to some extent in man, giving 

 rise to various rudimentary structures (see p. 211). From the standpoint 

 of its origin, therefore, there are three potential variables, the median and 

 the two ancestral lateral anlagen. The great changes in position, with 

 the development of the organs of the neck and thorax, introduce a second 

 opportunity for variation through incomplete or excessive caudad migra- 

 tion. And finally, the absence of a duct anchoring the gland, as it were, 

 to the alimentary canal, combined with the fact that the tissue can func- 

 tion equally well in almost any part of the neck region, favors the persist- 

 ence of variations. 



Blood Supply. The blood supply is very rich. The superior thyroid 

 arteries arising from the external carotids run to the apex of each lateral 

 lobe and give off numerous branches, which ramify over the ventral and 

 medial surfaces and penetrate into the substance of the gland. They 

 sometimes unite at the isthmus. Usually a dorsal branch anastomoses 

 with a twig from the inferior thyroid artery of the same side. Appar- 

 ently the anastomoses take place on the surface of the gland only. The 

 inferior thyroid arteries are as large or larger than the superior thyroids. 

 They spring from the subclavians and runs on the lateral surfaces of 

 the gland. They are intimately related to both the superior and in- 

 ferior parathyroid glands. Very frequently there is also a thyroidea ima 

 artery, which arises from the arch of the aorta and runs to the 

 isthmus. 



The blood leaves the thyroid through numerous veins which form a 

 dense plexus beneath the capsule. The superior thyroid veins are the 

 most constant. They leave the ventromesial surfaces of the lobes and open 

 into the facial or internal jugular veins. The middle thyroid veins run 

 laterally to enter the jugulars. They frequently anastomose with the 

 pharyngeal venous plexus. The inferior thyroid veins leave the caudad 

 aspect of the gland, and after forming a plexus on the ventral surface 

 of the trachea, pour into the left innominate vein. A thyroidea ima 

 vein, usually discharging also into the left innominate, may occur. All 

 the veins are said to be devoid of valves. 



Lymphatics. The lymphatics arise in networks about the follicles and 

 run along with the connective tissue septa to the surface of the gland, 

 where they form a very rich subcapsular plexus. While the chief drain- 

 age is to the deep cervical lymph glands, it may pass in almost any di- 

 rection, cephalad to the prelaryngeal nodes or caudad to the pretracheal 

 ones. Major has described, in the dog, the direct passage of one lym- 

 phatic trunk into the left subclavian vein, but it is not known whether 

 this ever occurs in man. 



