296 DAVID MARINE 



and left lo^es. Normally this tract between the foramen cecum and 

 thyroid isthmus undergoes complete absorption very early in fetal life, 

 leaving only the lateral lobes connected across the trachea anteriorly in 

 the region of the third and fourth trachea! rings by a flattened band of 

 thyroid one or two centimeters in width and less than half these measure- 

 ments in thickness. The human thyroid especially, although other mam- 

 mals as well, exhibits a wide range of abnormalities which in one way 

 or another center about the development, downgrowth and fate of the 

 thyroglossal tract. The most common of these abnormalities is the 

 presence of a pyramidal process. This is most frequently seen in asso- 

 ciation with endemic goiter and indicates an increased functional need 

 for thyroid tissue at the time when the tract should undergo absorption. 

 The pyramidal process usually replaces the isthmial band of which it is 

 the unabsorbed portion. There may be two pyramidal processes due to 

 the tract dividing high up. In that event there is a pyramidal process 

 extending obliquely downward and outward to each lateral lobe and an 

 absence of the isthmus. The pyramidal process may connect directly with 

 the right or left lobes, more frequently with the left. Occasionally there 

 is complete absorption of the isthmus as in many lower animals, e.g., dog, 

 sheep, etc., leaving the two lobes isolated. Earely one of the lobes (more 

 frequently the right) is rudimentary or absent. This is due to the 

 failure of the thyroid anlage to divide. Similar anomalies are seen in 

 the lung. Of greater importance are those cases where, the descent of 

 the thyroid "anlage" is arrested above the hyoid bone. In such instances 

 there is usually a large thyroid mass beneath the foramen cecum and 

 no thyroid tissue below the hyoid. Such cases unfortunately have been 

 operated upon for the removal of so-called sublingual goiters. There 

 may, however, be thyroid tissue around the foramen cecum and in in- 

 terrupted masses or in a continuous column from the foramen through 

 the hyoid hone and terminating in the pyramidal process co-existent with 

 well formed lateral thyroid lobes. This condition a^ain is more fre- 



t/ o 



quently seen in association with endemic goiter. These more or less iso- 

 lated masses of thyroid tissue along the line of descent of the thyroglossal 

 tract have been designated as "lingual," "sublingual/' "suprahyoid" and 

 "infnilmnd" thyroids. Born, Chamisso de Boucourt, Mayo, C. H. (&), 

 Streckeisen. 



Complete absence of the thyroid anlage occurs. All these variations 

 in the development, descent, bifurcation of the tract and the failure of 

 the thyroglossal tract to undergo normal absorption have been of great 

 aid in the development both of our knowledge of the embryology of 

 the thyroid and of the structures parathyroids, thymus and postbranchial 

 bodies derived from the branchial clefts. Thyroglossal cysts are formed 

 trom the thyroglossal duct by partial atresia or by an arrest of its 

 physiological absorption. Two types are seen: (1) those lined with 



