THE THYROID GLAND 



By R. K. S. LIM, M.B., Ch.B., 



Lecturer in Histology, University, Edinburgh. 



The thyroid is one of the few secreting glands which are 

 devoid of ducts, and which are consequently unable to pour 

 their secretion on the surface in the manner of such glands 

 as the salivary. Glands of this type are known as " internally 

 secreting " (Claude Bernard) or " endocrine " (Schafer), since 

 the secretion elaborated is passed into interior channels or 

 blood-vessels. 



Structure. — ^Arising in the first instance from a pharyngeal 

 outgrowth, the thyroid ultimately loses its original connection 

 and develops into a gland composed of closed rounded vesicles. 

 The vesicles, however (and the cells which line them), vary in 

 shape and size according to functional conditions. They 

 almost always contain a viscid fluid (colloid) which is the 

 product of their secretion. This, as explained above, is ab- 

 sorbed into the numerous blood-vessels supplied to the gland. 

 In addition to blood-vessels, the thyroid receives branches 

 from the autonomic (sympathetic and vagus) nerves. The 

 structure varies slightly in different animals, and accessory 

 thyroids are not uncommon. 



Lying behind each lobe of the thyroid, and even embedded 

 within its substance, are two or more small bodies, composed 

 of a compact mass of cells, known as the parathyroids. These 

 were first described by Sandstrom (1880), although the in- 

 ternal (embedded) parathyroids were unknown until they were 

 recognised by Kohn in 1895. 



Early History. — ^The function of the thyroid remained 

 obscure from the time of Galen (a.d. i 31-201) down to about 

 a hundred years ago. Galen taught that its secretion lubri- 

 cated the larynx ! His teaching was perpetuated by his suc- 

 cessors until the eighteenth century, when Haller (1776), recog- 

 nising the ductless nature of the gland, reasoned (after Ruysch) 

 that its secretion was carried away by the venous blood. 

 Simon (1884) made no advance on Haller when he suggested 

 that, as the thyroid was extremely vascular, it served as a 

 reservoir to regulate the flow of blood in the cranial cavity. 



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