1442 



THE DUCTLESS GLANDS 



Structure. The structure of the parathyroids is different from that of the thyroid. They 

 are composed of groups of epithelial cells arranged in a chain-like fashion with numerous inter- 

 vening capillaries. There is a certain type of cell, but the form varies. These variations result 

 from changes due to episodes of rest and activity (Verebely). MacCallum's studies seem to 

 lead to the same conclusion. Thompson 1 states that he finds only one type of cell in the infant 

 gland, and that in the adult there is primarily but one type of cell, the other cells noted being 

 modifications of the principal cell due to degeneration or hyperfunction. There is much lym- 

 phoid tissue in the interstitial connective tissue. The capillaries are of the sinusoidal variety. 

 The nerves of the parathyroids are derived from the sympathetic system. Each parathyroid 

 gland is supplied by a parathyroid artery. The inferior parathyroid artery is always a branch 

 of the inferior thyroid artery or of the anastomosing channel between the superior and inferior 

 thyroid arteries of one side (Geist). The superior parathyroid artery may be a branch of the 

 superior thyroid (Poole), but it is usually a branch of the inferior thyroid or of the anasto- 

 mosing channel. Ginsburg 2 has shown that each of the glands has an accessory blood supply, 

 by anastomotic channels from the opposite side. 



Embryology. The parathyroids develop chronologically in advance of the thyroid. They 

 are derived from the third and fourth branchial clefts of each side. An independent accessory 

 parathyroid may develop from the fifth cleft (Getzowa, Michand). Some have regarded the para- 

 thyroids as embryonic portions of the thyroid, but, as MacCallum says, " there is no histological 

 proof that parathyroid tissue can ever become converted into thyroid tissue." Most observers 

 regard the parathyroids as distinct glands possessed of a special function. Certain it is, as 

 Gley 1 and others have shown, removal of the parathyroids from herbivora, leaving the thyroid 

 intact, is followed by spasms, tetany, etc., just as complete thyroidectomy is followed by such 

 symptoms in carnivora. 



Applied Anatomy. Surgeons have become convinced that removal of the parathyroids in 

 man causes tetany, and that damage to them may produce serious symptoms. Because of this 

 danger most surgeons now prefer to remove a goitre from within the capsule of the thyroid gland, 

 after the plan of the Mayos, and thus avoiding the parathyroids. 



THE THYMUS GLAND (Fig. 1135). 



The thymus gland is a temporary organ, attaining its full size at the end of the 

 second year, when it ceases to grow and remains practically stationary until 

 puberty, at which period it rapidly degenerates. It does not entirely disappear, 



Trachea. 

 Thyroid veins. 



Right vagus. 



Superior vena cava. 



~% Thyroid body. 



w 



Left common carotid artery. 

 Left internal jugular vein. 



Left subdavian vessels. 



FIG. 1197. The thymus gland of a full-time fetus exposed in situ. 



for the shrunken and degenerated mass, even later in life, maintains a likeness to 

 the original form and retains within its substance small portions of thymus tissue 

 (Waldeyer). If examined when its growth is most active, it will be found to con- 



1 American Journal of the Medical Sciences, 1907, n. s.. cxxxiv p 562 

 ' University of Pennsylvania Medical Bulletin, January, 1908 

 Internal Secretions. By William Hanna Thomson. New York Medical Journal, November 19, 1904. 



