CHAPTER II 

 THE THYROID AND PARATHYROIDS 



THE thyro-parathyroid group of endocrine glands consists (1) of the thyroid 

 proper, which takes the form of two lobes situated on either side of the 

 larynx and windpipe, and often united by 

 an isthmus over the trachea (fig. 1); (2) 

 of the parathyroids : generally two on 

 each side one, the upper (parathyroid IV ), 

 which is usually in contact with the corre- 

 sponding lobe of the thyroid near its upper 

 and dorsal aspect, whilst the other, the 

 lower (parathyroid III), lies either in con- 

 tact with the corresponding lobe of the 

 thyroid near its lower and mesial aspect 

 or is removed a greater or less distance 

 from it, sometimes as low down as the 

 thymus. Both upper and lower para- 

 thyroids are usually supplied with blood 

 by branches of the inferior thyroid artery, 

 with which they are anatomically closely 

 connected. But there is a good deal of 

 variation in this, as well as in their posi- 

 tion relatively to the thyroid and to one 

 another (Halsted and Evans, Geis). In 

 many animals (dog, cat, rabbit, monkey) 

 the upper parathyroid is deeply embedded 

 in the substance of the thyroid (Kohn), and 

 it has on that account then been termed 

 the "internal" parathyroid, the lower being 

 designated " external." The expressions 

 parathyroid III and parathyroid IV, which 

 indicate the source of each gland, are on 

 the whole the least ambiguous and ought 

 if possible to be employed. 



FIG. 1. Sketch of a preparation of 

 human larynx and oesophagus from 

 the dorsal aspect, showing the thy- 

 roid and parathyroids. 



The thyroid may also contain a portion of tissue resembling thymus 

 gland, and, like that, containing lymphocytes and corpuscles of Hassall. 

 Such portion of thymus tissue is developed from the same branchial pouch 

 (IV) as the upper parathyroid, whereas the main thymus is developed 



