THYROID GLAND 



385 



The posterior surface is concave and is applied to the larynx and trachea. 

 It presents two lateral lobes, each with a rounded thickened base below 

 and a long pointed extremity extending upward, the lobes being con- 

 nected by an isthmus (see Fig. 79, page 388). Each of these lobes is 

 about two inches (50 millimeters) in length, three-quarters of an inch 

 (19 millimeters) in breadth, and about the same in thickness at its thick- 

 est portion. The isthmus connects the lower portion of the lateral lobes, 

 covers the second and third tracheal rings and is about half an inch 

 (12 millimeters) wide and one-third of an inch (8.5 millimeters) thick. 

 From the left side of the isthmus, and sometimes from the left lobe, is 

 a portion projecting upward, called the pyramid. The weight of the 

 thyroid gland is 350 to 380 grains (22 to 24 grams). Its weight in pro- 

 portion to the weight of the adjacent organs does not vary with age. 

 It is a little larger and more prominent in the female than in the male. 



Structttre of the Thyroid Gland. The thyroid gland is covered with 

 a thin but resisting coat of ordinary fibrous tissue which is loosely con- 

 nected with the surrounding parts. From the internal surface of this 

 membrane are fibrous bands, or trabeculae, giving off, as they pass 

 through the gland, secondary trabeculae and then subdividing until 

 they become of microscopic size. By this arrangement the gland is 

 divided up into small communicating cavities. The trabeculae contain 

 many small elastic fibres. Throughout the substance of the gland, 

 lodged in the meshes of the trabeculae, are rounded or ovoid closed 

 vesicles, measuring -\- to -%\- of an inch (40 to 100 /z). These are 

 formed of a structureless membrane and are lined with a single layer 

 of pale granular nucleated cells, 3"oVo to ^oVo ^ an ^ ncn (8 to 12 /z) 

 in diameter. The layer of cells sometimes lines the vesicle completely, 

 sometimes it is incomplete and sometimes it is wanting. The contents 

 of the vesicles are a clear, yellowish, slightly viscid, colloid sub- 

 stance, with a few granules, pale cells, and nuclei. The vesicles are 

 arranged in the form of lobules, and between them are the great veins 

 (see Plate IX, Fig. 4). 



Four small bodies, two on either side, lie posteriorly to the thyroid. 

 These are called parathyroids. Sometimes two are enclosed in the thy- 

 roid itself. They are composed mainly of cells resembling epithelium 

 but present no acinous vesicles. Accessory thyroids in the neck and 

 sometimes extending into the thorax have also been described. These 

 structures, however, have little more than an anatomical interest, except 

 that extirpation of the parathyroids may be followed by nearly all the 

 phenomena resulting from removal of the thyroid, and when death does 

 not follow thyroidectomy, this result is supposed to be due to the existence 

 of accessory thyroids. 



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