CHAPTER LXXXIII 

 THE THYROID AND PARATHYROID GLANDS 



Structural Relationships 



The thyroid and parathyroid glands are intimately associated, anatom- 

 ically, in most animals. The thyroid is present in all the vertebrates, 

 but the parathyroids do not occur below the amphibia. The thyroid 

 exists as two lateral lobes joined over the trachea by the so-called isthmus. 

 The parathyroids are very much smaller, being four in number and 

 located in pairs on the posterior aspect of the thyroid lobes. The two upper 

 parathyroids are usually more or less embedded in the thyroid tissue, 

 whereas the lower ones are much more loosely attached to the thyroid; 

 indeed, in some animals they are quite separate from it and may be 

 located at a distance, as in the mediastinum. Accessory thyroid and 

 parathyroid glands are sometimes present in the tissues of the neck, or 

 in the anterior mediastinum, accessory parathyroids being common in 

 the rabbit and rat, and parathyroid tissue being present in the thymus 

 in 5 per cent of dogs (Marine 75 ). Before these anatomical relationships 

 were thoroughly w r orked out, there was much confusion in the interpre- 

 tation of the results following removal of one or the other gland. 



In their histological structure and embryological derivation, the two 

 glands are very different. The parathyroids are developed as an out- 

 growth from the third and fourth branchial pouches, and they are com- 

 posed of masses of epithelial-like cells, sometimes more or less divided 

 up into lobules or trabeculae by bands of connective tissue. The cells 

 contain granules, some of which are of a fatty nature. Sometimes col- 

 loid-like material is found between the cells, or it may be enclosed in 

 small vesicles not unlike those of -the thyroid, although usually consider- 

 ably smaller. The blood vessels are extremely numerous, and form 

 sinus-like capillaries, which come into close relationship with the epi- 

 thelial cells of the glands. Nerves also are abundant and pass both to 

 the vessels and to the secreting cells. The blood vessels are derived from 

 the inferior thyroid artery. 



The thyroid is developed by immediate outgrowth from the entoderm 

 lining the floor of the pharynx, at a level between the first and second 

 branchial pouches. Represented at first by a solid column of cells, 

 there very soon occurs a division at the lower end into two lateral por- 



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