CHAPTEB I. XXXI II 

 THE THYROID AND PARATHYROID GLAND 



Structural Relationships 



The thyroid and parathyroid glands are intimately associated, anatom- 

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

 1'iit the parathyroids do nol occur below the amphibia. The thyi 

 exists as two- lateral Lobes joined over the trachea by the BO-called isthmus. 

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

 located in pairs on the posterior aspecl 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 Borne 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 rabbil and pat, and parathyroid tissue being present in the thymus 

 in 5 per eenl of dogs (Marine : . Before these anatomical relationships 

 were thoroughly worked out, there was much confusion in the inter] 

 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 t ho 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 trabecular by hands 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 nol unlike those of the thyroid, although usually consid< ' 

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

 sinusdike capillaries, which come into close relationship with the epi- 

 thelial cells of the glands. Xerxes also ;ire ahiindant and pass both to 



the vessels and to the Becreting 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 p 



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