228 The Vertebrate Organ Systems 



THE THYROID GLAND 



The thyroid gland is a vascular mass of tissue located near the 

 junction of the larynx and trachea. In human beings, it consists of two 

 lobes, which are on the anterior surface of the trachea. One lobe is 

 located on either side of the trachea, and they are connected by means 

 of a narrow isthmus. Normally the gland weighs from 25 to 40 grams, 

 its size varying with the age, state of health, diet, and habitation of 

 the individual. 



The gland is well supplied with blood vessels ; in fact it has one 

 of the richest supplies of blood of any organ in the body. Two layers 

 of connective tissue surround it, the inner adheres very closely to the 

 surface, while the outer is continuous with the fascia of the region. 



Histologically, the gland is composed of a large number of prom- 

 inent spherical follicles filled with a colloidal material. Lining the 

 follicles is a single layer of cuboidal or low columnar epithelium which 

 secretes the hormone iodothyroglohulin containing the active principle 

 thyroxin. The colloidal material in the follicles is condensed iodo- 

 thyroglohulin. 



The Function of the Thyroid Gland. — In many respects, the 

 secretion of the thyroid gland may be considered to be one of the most 

 important of the hormones, for it affects every cell in the body. It 

 affects every cell due to the fact that in some way its hormone regulates 

 the rate of oxidation of foodstuffs to produce heat and energy. Due 

 to this all-pervading influence, the effects of either over- or under- 

 secretion of this important gland soon become noticeable. 



While the general function is to control the rate of metabolism of 

 the whole body, it has many secondary effects such as influence over 

 the rate of the heartbeat, rate of respiration, blood pressure, and fat 

 deposition. 



The Composition of Iodothyroglohulin. — There are but few 

 of the hormones whose composition is well known. Thyroxin, the 

 active principle of iodothyroglohulin, has the unique distinction of being 

 one of the few hormones that has actually been synthesized. Thyroxin 

 is an amino acid, and has an empirical formula of C1BH11O4NL. From 

 this formula, it can be seen that the element, iodine, is an essential 

 part of the molecule ; in fact it forms about 65 per cent of thyroxin. 

 The importance of the iodine in the molecule is further emphasized by 



