SPLANCHNOLOGY 





third of the thyroid cartilage; the base looks downward, and is on a level with the 

 fifth or sixth tracheal ring. Each lobe is about 5 cm. long; its greatest width is 

 about 3 cm., and its thickness about 2 cm. The lateral or superficial surface is con- 

 vex, and covered by the skin, the superficial and deep fasciae, the Sternocleido- 

 mastoideus, the superior belly of the Omohyoideus, the Sternohyoideus and Sterno- 

 thyreoideus, and beneath the last muscle by the pretracheal layer of the deep 

 fascia, which forms a capsule for the gland. The deep or medial surface is moulded 

 over the underlying structures, viz., the thyroid and cricoid cartilages, the trachea, 

 the Constrictor pharyngis inferior and posterior part of the Cricothyreoideus, 

 the esophagus (particularly on the left side of the neck), the superior and inferior 

 thyroid arteries, and the recurrent nerves. The anterior border is thin, and inclines 

 obliquely from above downward toward the middle line of the neck, while the 

 posterior border is thick and overlaps the common carotid artery, and, as a rule, 

 the parathyroids. 



The isthmus (isthmus gl. thyreoidea) connects together the lower thirds of the 

 lobes; it measures about 1.25 cm. in breadth, and the same in depth, and usually 

 covers the second and third rings of the trachea. Its situation and size present, 

 however, many variations. In the middle line of the neck it is covered by the skin 

 and fascia, and close to the middle line, on either side, by the Sternothyreoideus. 

 Across its upper border runs an anastomotic branch uniting the two superior 

 thyroid arteries; at its lower border are the inferior thyroid veins. Sometimes the 

 isthmus is altogether wanting. 



A third lobe, of conical shape, called the pyramidal lobe, frequently arises from 

 the upper part of the isthmus, or from the adjacent portion of either lobe, but 

 most commonly the left, and ascends as far as the hyoid bone. It is occasionally 

 quite detached, or may be divided into two or more parts. 



A fibrous or muscular band is sometimes found attached, above, to the body 

 of the hyoid bone, and below to the isthmus of the gland, or its pyramidal lobe. 

 When muscular, it is termed the Levator glandulse thyreoidese. 



Small detached portions of thyroid 



, tissue are sometimes found in the vicin- 



ity of the lateral lobes or above the 

 isthmus; they are called accessory thy- 

 roid glands (glandules thyreoidece acces- 

 sories) . 



Development. The thyroid gland is 

 developed from a median diverticulum 

 (Fig. 1175), which appears about the 

 fourth week on the summit of the tuber- 

 culum impar, but later is found in the 

 furrow immediately behind the tuber- 

 culum (Fig. 979). It grows downward 

 and backward as a tubular duct, which 

 bifurcates and subsequently subdivides 

 into a series of cellular cords, from 

 which the isthmus and lateral lobes of 

 the thyroid gland are developed. The 

 ultimo-branchial bodies from the fifth 

 pharyngeal pouches are enveloped by 



the lateral lobes of the thyroid gland; they undergo atrophy and do not form true 

 thyroid tissue. The connection of the diverticulum with the pharynx is termed 

 the thyroglossal duct; its continuity is subsequently interrupted, and it undergoes 

 degeneration, its upper end being represented by the foramen cecum of the tongue, 

 and its lower by the pyramidal lobe of the thyroid gland. 



Thyroid gland 



Parathyroids 



Thymus 



Thymus 



Ultimo-branchial body 



FIG. 1175. Scheme showing development of bran- 

 chial epithelial bodies. (Modified from Kohn.) /, 

 //, III, IV. Branchial pouches. 



