INTERNAL PEOTECTIVE SECRETIONS 



in structure (as well as function, infra) from the accessory thyroids. 



They were first described by Sandstrom (1880) in man and certain 



other mammals, externally 



to the lateral lobes of the 



thyroid body. Subsequent 



observations confirmed their 



constant presence in mam- 



malia, adding to the extern"/ 



parathyroids other similar 



little glands situated on the 



mesial surface of the lateral 

 lobes of the thyroid, the i> 

 internal parathyroids, which, 

 however, may be absent in 

 certain species (Fig. 3). 



In man the outer (also 

 called the inferior) para- 

 thyroids lie in front of the 

 inferior thyroid artery and 

 the recurrent nerve. Their 

 position is not constant. 

 For the most part they are 

 situated at the inferior 

 angle of the thyroid lobes, 

 towards the lower part of 

 the postero-external border, 



FIG. 3. Transverse section of left lobe of thyroid from 



at a greater 01" leSS distance a two-months' kitten. (Kolm.) , thyroid tissue ; 

 f , nil -j_ i ft, thymic tissue; ;> '. innor and outer iiara- 



froui it, and closely united thyroids. 



by fine connective tissue. 



More rarely they are found at the level of the eighth and tenth 



tracheal ring (Fig. 4). It follows that in excising the thyroid body 



in man by the subcapsular method, the inferior or outer para- 



thyroids are easily left in situ a fact which, as we shall see, is 



of great clinical and physiological importance. 



The inner (or superior) parathyroids are situated on the internal 

 surface, towards the upper pole of the thyroid lobes, with which 

 they are intimately connected, since they are wrapt in a common 

 sheath of connective capsular tissue, and sometimes lie in the 

 depth of the thyroid substance. In surgical thyroidectomy these 

 must obviously be excised along with the thyroid body. 



The structure of the parathyroids (both outer and inner) 

 differs from that of the principal and accessory thyroids. They 

 consist not of hollow vesicles, but of compact masses or columns 

 of epithelium cells, which sometimes anastomose into branching 

 cords. Between the cell masses there are septa of connective 

 tissue, which convey the blood-vessels and nerves into the gland 

 substance (Fig. 5). 



