THE NECK 169 



triangle, or they may enlarge in a downward direction and 

 pass through the thoracic inlet (intra-thoracic variety). 



Adenomata or solitary cysts of the thyreoid gland may be 

 removed by enucleation. In this operation the capsule and 

 the gland substance are incised until the wall of the cyst or 

 adenoma is reached, and the tumour is then removed by blunt 

 dissection, together with the thinned-out portions of glandular 

 tissue and sheath which lie superficial to it. 



Loss of the thyreoid secretion during childhood, either from 

 atrophy or congenital absence of the gland, leads to the condition 

 of cretinism, while atrophy or complete removal of the gland in 

 adult life causes myxcedema. 



The Parathyreoid Glands are small pea -like structures, 

 two on each side, which lie normally on the posterior aspect 

 of the lateral lobes of the thyreoid gland, and are most 

 commonly found embedded in its fibrous capsule. The 

 removal of all of these bodies leads to tetany and death. 

 Consequently, in total removal of the thyreoid gland, which 

 is only performed for malignant disease, great care is exercised 

 to leave some of the parathyreoids intact. They can usually 

 be identified by their close relation to an anastomosing 

 vessel which connects the superior and inferior thyreoid 

 arteries on the posterior aspect of the lateral lobe of the 

 gland. 



Development of the Thyreoid Gland. The thyreoid gland 

 arises as a median diverticulum of the entoderm of the floor of 

 the primitive pharynx, and grows tailwards in the interval 

 behind the tuberculum impar (p. 193). In doing so it passes 

 ventral to the cartilages of the second to the sixth visceral arches. 

 At first a hollow bud, the thyreoid rudiment soon becomes solid, 

 and enlarges to form the isthmus and lateral lobes of the gland. 

 Its connection with the floor of the mouth, which is termed the 

 thyreo-glossal duct, gradually disappears, and the foramen caecum 

 on the dorsum of the tongue is all that normally remains of its 

 upper end. The pyramidal lobe and the levator glandulse 

 thyreoidese are derived from its lower end. 



Occasionally the thyreo-glossal duct is found embedded in 

 the hyoid bone. It is probable that in these cases the diver- 

 ticulum passed behind the cartilage of the second but in front 

 of the cartilage of the third arch, and was caught between them 

 when they fused to form the body of the bone (p. 149). Abnormal 

 remains of the duct may form thyreo-glossal cysts and tumours. 



