DEVELOPMENT OF THE PHARYNX AND NECK. 47 



detached part of the thyroid-hyoid muscle — the levator glandulae 

 thyroideae. The body of the hyoid bone is developed in the track 

 of the thyro-glossal duct (Fig. 33) and splits it up. Eemnants 

 of the duct or of secondary detached acini of the thyroid 

 may persist and form cysts in the base of the tongue above 

 the hyoid, and commonly between the genio-glossus muscles. 

 They may also occur between the hyoid and thyro-hyoid mem- 

 brane. The supra-hyoid or infra-hyoid bursae may also become 

 cystic, and may be mistaken for thyro-glossal cysts. 



The lateral lobes of the thyroid are developed from the inner 

 recess of the 4th cleft, the position of which is marked in the 

 adult by the pyriform fossae (see Figs. 33 and 34). These 

 pockets, like the thymic of the 3rd cleft, soon lose their connection 

 with the hypoblastic lining of the pharnyx, and become isolated 

 buds which divide and re-divide until a collection of isolated 

 acini is formed. The lateral lobes come in contact as they grow, 

 with the median (lingual) lobe under the laryngeal and tracheal 

 groove in the floor of the primitive pharynx. As they grow 

 outwards the thyroid buds come in contact with the cervical sinus 

 (see page 32), and at one time were supposed to spring from the 

 epiblastic lining of the sinus. They receive their blood supply 

 from the 4th arch (inferior thyroids), while the median lobe 

 is mainly supplied from the ventral aortae, between the 2nd and 

 3rd arches (superior thyroids). The nerve-supply comes on its 

 arteries from the superior and middle cervical ganglia of the 

 sympathetic. Its nerves appear to have their origin in the upper 

 dorsal segments of the spinal cord. 



In the process of development minor buds of the thyroid may 

 become detached. These form accessory thyroid bodies. 



In manner of origin and growth the thyroid resembles the ton- 

 sil and thymus, but unlike these it is not transformed into a 

 lymphoid structure. 



Para-thyroids. — These vary in number from three to five on 

 each side (Welsh) and are small bodies of a brownish-red 

 colour, and measuring 6 to 8 mm. in diameter. One or two are 

 situated on the outer side of each lateral lobe amongst the 

 branches of the superior thyroid arteries. They arise with the 

 bud of the thymus from the third cleft. One or two occur 

 constantly on the tracheal aspect of each lateral lobe, amongst the 



