CHAPTER LXXXVI 



THE THYROID AND PARATHYROID GLANDS 

 Structural Relationships 



The thyroid and parathyroid glands are intimately associated, anatomically, 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 num- 

 ber 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, where as lower ones 

 are much more loosely attached to the thyroid; indeed, in some animals, particularly the 

 herbivora, 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 35 ). Before these anatomical relationships were thoroughly worked 

 out, there was much confusion in the interpretation of the results following removal of 

 one or the other gland. 



In their histologic structure and embryological derivation, the two glands are very 

 different. The parathyroids are developed as an outgrowth from the third and fourth 

 branchial pouches, and they are composed of masses of epithelial-like cells, sometimes 

 more or less divided up into lobules or trabeculse by bands of connective tissue. The 

 cells contain granules, some of which are of a fatty nature. Sometimes colloid-like ma- 

 terial is found between the cells, or it may be enclosed in small vesicles not unlike those 

 of the thyroid, although usually considerably smaller. The blood vessels are extremely 

 numerous, and form sinus-like capillaries, which come into close relationship with the 

 epithelial 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 portions, and the original solid column becomes hollowed out. The two 

 lateral branches of the original column divide again and again so as to form a system of 

 hollow tubes lined with epithelium. These afterward become cut up. so as to form the 

 closed vesicles characteristic of the gland. Each vesicle is more or less spheroidal in 

 shape, and has no basement membrane, but its walls are formed by a layer of epithelial 

 cells, which may be columnar, cubical, or flattened in shape. Each vesicle is filled with 

 the so-called colloid material, which is peculiar in containing iodine, and between the 

 vesicles is a layer of connective tissue often containing small cells, some of which are 

 not unlike those of the parathyroid. The connective tissue also contains the blood ves- 

 sels, which are very numerous indeed, the thyroid, in proportion to its size, receives 

 more than five times as much blood as the kidneys, the only tissue that surpasses it in 

 this regard being the medulla of the adrenal gland (see page 211). The nerves arise 

 from both the vagus and the sympathetic systems and have been traced to the secreting 

 epithelial cells. The above description applies to a strictly normal gland. 



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