552 



COLLEGE ZOOLOGY 



thyroxin have been synthesized by chemists. 

 The hormones now sold are either syn- 

 thesized or come from mammals. Insulin, 

 which makes it possible for diabetics to live, 

 is extracted from beef pancreas. 



Because of the similarity of the action of 

 hormones in most vertebrates, the following 

 discussion will deal with the human endo- 

 crine system. The most important of the 

 ductless glands in man are the thyroid, para- 

 thyroids, adrenals, pituitary, islets of Lan- 

 gerhans, and gonads (Fig. 398). Hormones 

 are also produced by the membrane lining 

 the intestines, and possibly the stomach. 

 There is still no convincing evidence of 

 endocrine function in the pineal and thymus 

 glands. 



Thyroid 



The thyroid is a bilobed gland, which 

 weighs about an ounce and lies in front of 

 the trachea below the larynx; it influences 

 the general rate of metabolism in the body. 

 It secretes a hormone called thyroxin, which 

 contains about 65 per cent iodine by weight. 

 When too little thyroxin is secreted, the 

 thyroid may become much enlarged to form 

 a goiter. When the thyroid fails to function 

 properly in early life, children may become 

 dwarfed and mentally deficient, a condition 

 known as cretinism. In goiter districts, sup- 

 plemental iodine should be supplied, espe- 

 cially in the diet of pregnant women. This 

 is frequently accomplished by the use of 

 iodized salt. If the thyroid of an adult 

 atrophies or is removed, myxedema occurs, 

 which results in the subcutaneous tissues 

 becoming infiltrated with mucoid substance, 

 causing the face and hands to swell. Over- 

 activity of the thyroid or hyperthyroidism 

 is known as exophthalmic goiter or Grave's 

 disease. Thyroxin has been chemically iso- 

 lated and crystallized, and also synthesized. 



Parathyroids 



The parathyroids are 4 small glands on 

 the posterior surface of the thyroid. They 



produce a secretion which contains a hor- 

 mone that exercises a profound influence on 

 the metabolism of calcium and phosphorus 

 in the body. A deficiency of this hormone 

 results in low blood calcium and violent 

 twitching of the muscles, a condition known 

 as tetany; an excess, as in parathyroid 

 tumors, may cause excessive withdrawal of 

 calcium from the bones. 



Adrenals 



The adrenals are 2 small glands perched 

 like cocked hats on top of the kidneys. Each 

 consists of 2 parts, the cortex and medulla. 

 The cortex secretes about 30 known com- 

 pounds. Chemically, these are steroids. One 

 of the best known of these steroids is corti- 

 sone, which has proved beneficial in some 

 types of arthritis. Removal of the adrenal 

 cortex causes death. 



Injury to the cortex of the adrenals re- 

 sults in Addison's disease, which is charac- 

 terized by anemia, low blood pressure, in- 

 testinal disturbances, sometimes deep 

 bronzing of the skin, and ending in death if 

 not treated. 



The medulla secretes the hormone 

 epinephrine (adrenalin). It mimics the ac- 

 tion of sympathin which is secreted by the 

 tips of the nerves of the thoracolumbar 

 division of the autonomic nervous system. It 

 has been suggested that the secretion of 

 adrenalin constitutes a reserve mechanism 

 that comes into action at times of stress. 

 Under emotional excitement, such as fear 

 or anger, additional adrenalin is secreted, 

 and the individual is prepared for "fight or 

 flight" by acceleration of the heart, and blood 

 vessel constriction, which raise the blood 

 pressure; in addition, the blood sugar is 

 elevated by conversion of liver glycogen. 

 Adrenalin is widely used in treating asthma. 



Pituitary or hypophysis 



The pituitary gland (Fig. 398) is about 

 the size of a pea, and is located at the base 

 of the brain in a depression of the sphenoid 



