154 PRINCIPLES OF ANIMAL BIOLOGY 



hormone action are those exhibited by certain definite glands. These 

 gland.s do not have ducts, or, if they do, the hormone is not ejected 

 through the duct. All hormones diffuse directly into the blood. Such 

 ductless glands are known as endocrine glands, and the hormones are also 

 called endocrine secretions. The best-known hormonal actions are 

 those of man, so the account here given must draw heavily upon the facts 

 ascertained for human endocrine glands. The names and locations of 

 most of those which are known or believed to be endocrine are shown in 

 Fig. 12G. 



Endocrine Glands and Their Work. — One qf the best-known hor- 

 mones is that of the thyroid gland, a bilobed structure lying beneath and 

 beside the trachea in the neck in man. Its hormone, called thyroxin, 

 has been isolated and has the formula C15H11O4NI4. The direct effect 

 of thyroxin is to increase the rate of metabolism. Deficiency of this 

 hormone in children or young animals retards their development. If 

 this influence starts early enough it leads to cretinism, in which body and 

 limbs are dwarfed and distorted, and mental development is arrested. 

 Some regions of the Avorld have little iodine in the soil, hence little in 

 crops, and the inhabitants are finable to produce adequate thyroxin, 

 which includes that element. Cretins were common in such regions 

 until public health measures, such as the requirement that potassium 

 iodide (KI) be added to table salt, were adopted. Deficiency of thyroxin 

 in adults often causes endemic goiter, a swollen condition of the thyroid 

 caused by an accumulation of a colloid fluid in the capsules of the gland. 

 A more serious effect of deficient thyroid is myxedema, with its low 

 metabolism, a state of lethargy, and puffed skin. Excessive thyroxin 

 commonly causes exophthalmic goiter, with its increased metabolism, 

 high blood pressure, and protruding eyeballs; removal of part of the 

 thyroid, the proportion depending on how much the metabolic rate has 

 been raised, is one of the cures. 



Closely associated with the thyroid (imbedded in it in man) are the 

 parathyroids. There are four of these bean-shaped bodies in the human 

 thyroid. Separate experimentation with them has been hindered by 

 their position. Their primary effect is upon calcium and phosphorus 

 metabolism, and the calcium deposit in bones is reduced when the para- 

 thyroids are deficient. Complete removal of the glands causes violent 

 muscular convulsions. 



The adrenal glands rest on the kidneys (above them in man). They 

 consist of a central part or medulla, which arises in the embryo as an 

 outgiowth of the nervous S3^stem, and an outer part or cortex, which 

 (;omes from the lining of the coolom. The two parts produce different 

 hormones, that from the cortex being the more critically important. 

 About one-fifth of the cortex suffices for normal processes, but if the whole 



