THE HEAD AND NECK 1125 



The first indication of each lateral thyroid is an evagination of the ento- 

 derm of the corresponding loorth visceral cleft. This evagination is called 

 the lateral thyroid diverticulum. These lateral diverticula, hke the median, 

 form two thick-walled epithelizd vesicles, which soon become soUd, and lose 

 their connections with the fourth visceral clefts. Thereafter each fuses with 

 the corresponding lobe at the caudcd end of the median vesicle, and o}te solid 

 epithelial mass is thereby formed, which is invested by mesoderm, and which 

 lies across the laryngo-tracheal tube. The lateral thyroids, as stated, form 

 only small parts of the adult thyroid body. 



Connective tissue, derived from this mesodermic investment, now invades 

 the sohd mass, and it is broken up into numerous soUd epitheUal cords, which 

 anastomose freely, and so give rise to an intricate reticulum, the meshes of 

 which are occupied by connective tissue and bloodvessels, of mesodermic 

 origin. The sohd epithelial cords of the reticulum become hollow, and the 

 lumina, so produced, are broken up at intervals, by constrictions, into closed 

 vesicles, which contain the coUoid material. 



The pyramid of the thyroid body sometimes met with in connection with 

 the isthmus of the adult thyroid, is developed from the caudal part of the 

 thyro-glossal dact. 



The epitheUal ceUs of the vesicles of the adult thyroid are derived from 

 the entoderm of the pharyngeal part of the fore-gut. 



Parathyroid Bodies. — The parathyroids are four in number, and are arranged 

 in pairs. The upper pair are related to the dorsal borders of the lateral 

 lobes of the thyroid body, and the lower pair are placed behind the lower 

 ends of the lateral lobes. They are developed as evaginations of the ento- 

 derm of the third and fourth visceral clefts, of either side. The parathyroids 

 present no traces of closed vesicles for the colloid material. 



Accessory Thyroid Bodies. — These bodies are sometimes met in the neigh- 

 bourhood of the hyoid bone, and are known as the suprahyoid and prehyoid 

 bodies. They are developed as buds or evaginations of the thyro-glossal 

 duct, cind they consist of thyroid tissue. 



The Trachea and (Esophagus. 



Trachea. — The trachea extends from the cricoid cartilage of the 

 larynx to about the level of the disc between the bodies of the 

 fourth and fifth thoracic vertebrae, where it divides into the two 

 bronchi, right and left. Its average length is about 4^ inches, 

 and its width about i inch. Anterioriy and laterally it is cylindrical 

 and firm, but posteriorly it is flattened and membranous, so that it 

 does not press upon the oesophagus, over which it Ues. It occupies 

 a mesial position, and its direction is downwards with an incUnation 

 backwards. It is divisible into two parts, cervical and thoracic. 



For the trachea in the thorax, see p. 1019. 



The cervical part of the trachea extends from the cricoid cartilage 

 to the level of the upper border of the manubrium stemi, and it 

 measures about 2^ inches in length. It is freely movable, and is 

 surrounded by areolar tissue, which contains many elastic fibres, 

 and is somewhat loosely arranged. 



Relations — Anterior. — The isthmus of the thyroid body lies 

 directly upon the second and third rings as a rule, the first ring 

 usually lying exposed between its upper border and the cricoid 

 cartilage. Superficial to the first ring, on the left of the median 

 line, there may be the lower part of the p5rramidal, or middle lobe 

 of the thyroid body. Below the isthmus is the inferior thyroid 

 plexus of veins, from which the right and left inferior thyroid veins 



