MULTICELLULAR ANIMALS 203 



all regions, a condition that in part determines their rates of metab- 

 olism. (2) Actively, the blood serves as a correlating agent by dis- 

 tributing special substances, the hormones. 



The Endocrine Glands. Hormones. In recent decades it 

 has been found that, in higher animals at least, there is an intricate 

 system of special glands (endocrine glands) whose function it is to 

 release into the blood stream minute quantities of special chemical 

 agents that activate other tissues. These substances are termed hor- 

 mones; their study has become a separate branch of Biology known 

 as ENDOCRINOLOGY. Attention has focused chiefly on vertebrates; 

 comparatively little is known concerning hormones in invertebrates. 



The endocrine glands are sometimes called the ductless glands, 

 because they possess no ducts; their excretions are passed directly 

 into the blood from the cells by osmosis. Intensive study of the 

 chemical nature and structure of the hormones and of the efifects of 

 removal of such glands and subsequent treatment with gland ex- 

 tracts have brought about marked progress in the understanding 

 of body functions and in the control of disease. In the mammalian 

 body these glands are widely separated and their products act in 

 various ways to control many of the vital processes. 



The Thyroid. The chief endocrine glands are (Fig. 138) : The 



THYROID, the PITUITARY, the PANCREAS, the ADRENALS, the PARATHY- 

 ROIDS, and the gonads. The thyroid is a large gland of endodermal 

 origin situated in mammals in the base of the neck. Its product is 

 known as thyroxin, and is an iodine-containing compound, the 

 structure of which is known. In the frog tadpole thyroxin brings 

 about the metamorphosis from the water-living tadpole to the air- 

 breathing frog. In Man this hormone is a regulator of the rate of 

 oxygen utilization by the body; in other words it controls the pace 

 at which oxidative metabolism transforms energy. When the pro- 

 duction of thyroxin is excessive the result is an increase in the rate 

 of oxidative metabolism accompanied by general body disturbances. 

 If the thyroid is deficient during early life, the individual may de- 



