THE THYROID BODY. 1793 



plexus beneath the capsule, from which the blood escapes by three chief courses on 

 each side. The superior thyroid veins are double, and follow the artery to open either 

 into the internal jugular directly or into the facial. They may communicate with 

 the linguals. The middle thyroid vein, less regular, passes from the side of the lobe 

 into the internal jugular, anastomosing, as a rule, with the pharyngeal venous plexus. 

 The inferior thyroid veins, generally two in number, some 5 mm. in diameter, come 

 from the deeper part of the organ and form a rich plexus in front of the trachea under 

 the middle layer of the cervical fascia, draining, for the most part, into the left in- 

 nominate ; but a vein may end at the angle of the two innominate veins. The in- 

 ferior thyroid veins can be injected from below. 



The lymphatics begin within the organ as perifollicular lymph-spaces ; from 

 these plexuses follow the interlobular septa in their course to the exterior, where they 

 constitute a superficial plexus from which the lymph passes in all directions. Some 

 run upward from the isthmus to small lymph-nodes in front of the larynx, some 

 from the sides to the deep glands about the internal jugular, and some from the 

 isthmus and adjacent parts downward to pretracheal lymph-nodes. 



The nerves are derived, for the most part, from the cervical sympathetic. It 

 is probable that filaments are contributed by sympathetic fibres running in company 

 with the inferior laryngeal and the hypoglossal nerves. In addition to the fibres 

 destined for the walls of the blood-vessels, the terminal twigs end around the follicles 

 in close relation with the glandular epithelium. 



Development. The thyroid is developed from an unpaired median anlage. 

 This anlage, irregular in form and size, (Fig. 1521), appears in embryos of from 

 3-4 mm. as an epithelial outgrowth from the anterior wall of the primitive pharynx 

 in the region of the second visceral arch, and therefore in close relation with the 

 posterior part of the tongue. At first possessed of a narrow lumen, the evagination 

 soon loses its cavity and becomes a solid pyriform mass, which for a short time is 

 connected with the pharyngeal wall by a delicate epithelial strand. Usually the latter 

 soon disappears and the isolated median anlage, which meanwhile rapidly increases 

 as a bilobed mass, passes to the lower level of the early thyroid. The position of 

 the primary outgrowth is later indicated by the depression on the tongue, the fora- 

 men c&cum, just behind the apex of the V-row of the circumvallate papillae. Occa- 

 sionally the evagination persists, and then forms the thyro-glossal duct, a narrow tube 

 extending for a variable distance from the tongue towards the thyroid body. The 

 epithelial outgrowth, which, on either side, appears on the ventral wall of the 

 fourth pharyngeal furrow, was formerly known as the lateral thyroid anlage, 

 under the belief that it contributed to the formation of the lateral lobe of the thyroid 

 gland. Later studies have shown, however, that this is not the case, the outgrowths 

 in question usually disappearing, or giving rise to small masses resembling thymic 

 tissue. Mention may be made of a pair of outgrowths from the floor of the primitive 

 pharynx where the fifth furrow would be. These are the ultimo -branchial bodies, 

 which usually degenerate and disappear. 



The histogenesis of the thyroid includes two stages, the first being distinguished 

 by numerous cylindrical epithelial cords from which grow out lateral branches. The 

 second stage witnesses the fusion of these epithelial cords into a net-work the meshes 

 of which are occupied by vascular mesoblastic tissue. During the third foetal month 

 the epithelial reticulum breaks up into masses corresponding to the follicles of the 

 thyroid. These gradually acquire a lumen around which the cells become arranged 

 to constitute the epithelial lining of the compartments in which later the characteristic 

 colloid substance is secreted. The thyroid agrees with the parathyroids and the 

 thymus in originating from the walls of the primitive pharnyx and, likewise, in devi- 

 ating in its later development from its primary correspondence to a typical gland. 



Accessory Thyroids. Small detached bodies of the same structure as the 

 thyroid are occasionally found about the hyoid bone in the median line, both before 

 and behind and sometimes below it. They are remnants of the median thyroid diver- 

 ticulum from the primitive pharynx, sometimes represented by the thyro-glossal duct. 

 This passed originally in front of the hyoid bone, thus accounting for suprahyoid 

 and prehyoid accessory thyroids. Those behind and below the hyoid are probably 

 the result of an upward or downward growth from the primary diverticulum. 



