212 GENERAL ZOOLOGY 



disappear, the follicles shrink in size and the height of the 

 follicle cells increases. Thus under the stress of pituitary 

 stimulation the follicles are emptied and the additional secretion 

 produced by the stimulated follicle cells is so rapidly removed 

 from the glands that the follicles remain empty. This is also the 

 picture one finds in cases of hyperthyroidism. On the other 

 hand, in cases of endemic goiter (hypothyroidism) the follicle 

 cells are low in height and the follicles are filled with colloid, 

 which, however, is without the potency of the normal colloid 

 because of insufficient iodine. In a normal individual, the 

 colloid and the thyroid hormone contained in it seem to accumu- 

 late in the follicles under normal conditions of thyroid activity 

 to produce a reserve supply of hormone. When the gland is 

 stimulated, the colloid is resorbed by the follicle cells and passes 

 into the blood. The height of the cells forming the walls of the 

 follicles is a direct index of the activity of the gland. 



Parathyroid Gland. — In man the parathyroid glands consist 

 usually of two pairs of nodules attached to, or embedded in, the 

 dorsal surface of the thyroid. They consist of corded masses of 

 polygonal cells, with very little connective tissue. The follicles 

 so characteristic of the thyroid gland are lacking in the para- 

 thyroid. Complete removal of the parathyroids in dogs is 

 followed by death in four days, with the development of tetany 

 (spasmodic muscular contraction) and a fall of calcium in the 

 blood. Injection of calcium salts causes the tetany to disappear 

 and partially restores the animal to normal conditions. Extracts 

 of parathyroid gland when injected produce results similar 

 to calcium salts. It is therefore concluded that the parathyroids 

 produce a hormone which regulates calcium metabolism and 

 maintains a normal calcium content in the blood. Since removal 

 of the anterior pituitary lobe also produces disturbances in 

 calcium metabolism, there is some evidence that a parathyro- 

 tropic hormone is produced by the pituitary which has a relation 

 to the parathyroid similar to the relation of the thyrotropic 

 hormone to the thyroid. 



Thymus Gland. — The thymus gland of mammals extends down 

 from the lower throat region into the thorax, overlying the 

 pericardium. Like the thyroid and parathyroid glands, it is also 

 a derivative of the embryonic pharynx. It increases in size until 

 puberty, after which it undergoes atrophic changes so that in the 



