II24 



A MANUAL OF ANATOMY 



Nerves. — These are derived from the sympathetic plexuses 

 which accompany the superior and inferior thyroid arteries. 



Lymphatics. — The lymphatic vessels of the thyroid body are 

 disposed in two groups — ascending and descending. The ascending 

 lymphatics form three sets — median and two lateral, right and left. 

 The median ascending lymphatics return lymph from the upper part 

 of the isthmus, and pass to the prelaryngeal glands. The. lateral 

 ascending lymphatics, on either side, accompany the superior 

 thyroid artery, and pass to the inferior deep cervical glands on a 

 level with the cricoid cartilage of the larynx. The descending 

 lymphatics also form three sets — median and two lateral, right and 

 left. The median descending lymphatics return lymph from the 

 lower part of the isthmus, and pass to the pretracheal glands. The 

 lateral descending lymphaiics, on either side, accompany the inferior 



thyroid artery, and pass to the in- 

 ferior thyroid glands, which lie in the 

 groove between the trachea and oeso- 

 phagus, the efferents of which ter- 

 minate in the inferior deep cervical 

 glands. 



Structure. — The thyroid body has an 

 external capsule of dense connective tissue 

 which sends trabeculae into the interior, 

 thereby dividing it into irregular lobules. 

 These lobules are composed of groups of 

 closed vesicles, which are connected to- 

 gether by areolar tissue. The vesicles are 

 oval or spherical, and each is lined with 

 a single layer of columnar, or cubical, epi- 

 thelium . They contain a yellowish, viscid, 

 Fig. 458. — Section of the albuminous fluid called colloid, and are 

 Thyroid Body, showing the surrounded by networks of capillary blood- 

 Vesicles and their Epi- vessels. 



Development. — The thyroid body is de- 

 veloped from the entoderm of (i) the 

 ventral wall of the pharyngeal portion 

 of the primitive gut, and (2) the fourth 

 visceral clefts. It consists of three parts — namely, a median thyroid, and 

 two lateral thyroids. The median thyroid represents (i) the isthmus, and 

 (2) the greater parts of the lateral lobes, of the adult thyroid, whilst the lateral 

 thyroids represent only small parts of the lateral lobes of the adult thyroid. 



The first indication of the median thyroid is an evagination of the ventral 

 pharyngeal entoderm between the ventral ends of the second visceral clefts, 

 immediately behind the tuberculum impar. This evagination is called the 

 median thyroid diverticulum. It forms a thick-walled epithehal vesicle, 

 embedded in mesoderm, which soon becomes solid. As the vesicle grows in 

 a downward direction over the ventral aspect of the primitive larynx, its 

 distal end becomes bilobed. Superiorly it retains for some time its com- 

 munication with the ventral wall of the pharynx, behind the tuberculum 

 impar, by a hollow pedicle, which constitutes the thyro-glossal duct. This 

 duct usually disappears, its superior orifice being represented on the dorsum 

 of the adult tongue by the blind recess, called the foramen cacum. In very 

 rare cases the lingual portion of the duct may persist for a short distance, 

 in which cases the foramen caecum leads to the lumen of a short tube, known 

 as the ductus lingualis. 



The median thyroid, as stated, gives rise to the isthmus and greater parts 

 of the lateral lobes of thfe adult thyroid body. 



thelial Lining. 

 (The Colloid is indicated.) 



