428 METABOLISM, NUTRITION, AND DIET 



thyroid is covered by the muscles of the neck. It is highly vascular, and 

 varies in size in different individuals. 



The gland is encased in a thin transparent layer of dense areolar tissue, 

 free from fat, containing elastic fibers. 



These gland vesicles are each lined with a single layer of cubical cells and 

 are filled with transparent nucleo-albuminous colloid material. 



Accessory Thyroids. The accessory and the parathyroids possess 

 the structure of the thyroid and apparently perform the same function. The 

 accessory thyroids undergo hypertrophy when the thyroid has been removed. 



The colloid material which is formed within the thyroid vesicles, and is 

 believed to be their secretion, finally ruptures through their walls into the 

 lymph channels and thus gains entrance to the circulation. The secretion 

 of the thyroid falls intq> the class known as internal secretions, and exerts a 

 profound influence upon the metabolic processes of the body, probably 

 through its influence on the central nervous system. Complete extirpation 

 of the thyroid, at least in some animals, produces death, preceded by a group 

 of characteristic symptoms. In man and the monkey the symptoms after re- 

 moval come on slowly and resemble the disease known in man as myxedema. 



This disease is known definitely to be due to disease of the thyroid, where- 

 by its function is interfered with. Moreover, if a piece of thyroid of sufficient 

 size be grafted into an animal from which the glands have been removed, 

 and the graft takes, the symptoms of thyroid removal are lessened in inten- 

 sity or disappear altogether. Thyroid feeding or the administration of 

 thyroid extracts relieves the symptoms of the disease myxedema. 



The above facts show that the thyroid gland must perform some im- 

 portant function in the animal economy, and it is believed that this is accom- 

 plished by virtue of its internal secretion. The colloid material of the gland 

 has been submitted to much chemical study, and a substance called iodo- 

 thyrin has been isolated as its active principle. Baumann and Roos state 

 that iodothyrin exists in the gland in combination with proteid bodies. lodo- 

 thyrin relieves the symptoms of thyroid removal much to the same extent 

 as thyroid feeding. It is a very resistant substance, and is not injured by the 

 action of the gastric juice or by boiling with 10 per cent sulphuric acid for 

 a long time. 



The Suprarenal Capsules or Adrenals. These are two flattened, 

 more or less triangular or cocked-hat shaped bodies, resting by their lower 

 border upon the upper border of the kidneys. 



The gland tissue proper consists of an outside firmer cortical portion, and 

 an inside soft dark medullary portion, figure 312. 



The adrenals are very abundantly supplied with nerves, chiefly com- 

 posed of medullated fibers. These fibers are derived from the solar and renal 

 plexuses and the vagi, but the method of their termination is unknown. 



A vast amount of information has been given concerning the function of 



