DIGESTION 367 



into sugar and cast into the blood. In the Uver also the ammonia 

 which results from the decomposition of proteins is converted 

 into urea ready for excretion by the kidneys, and various other 

 chemical changes take place. 



Ill I i-innii 



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ENDOCRINE AND OTHER ORGANS 



We must here mention the organs known as the ductless 

 glands or endocrine organs, which, while they manufacture 

 substances of importance to the body, 

 discharge these products not through a 

 duct but into the blood or lymph by the 

 process known as internal secretion. Sub- 

 stances are, of course, continually being 

 passed into the blood by every organ, but 

 among these it is important to distinguish 

 between (a) the waste products of meta- 

 bolism, {b) substances which are used in 

 bulk, such as the sugar discharged by the 

 liver, and (c) the ' chemical messengers ' 

 or hormones (p. 18), of which minute 

 quantities are secreted as a stimulant or 

 inhibiting agent to the action of other 

 organs. It is to this latter class that the 

 characteristic products of the ductless 

 glands belong. The functions of the ductless 

 glands are discussed in Chapter 24. 



The thyroid glands of the frog are a 

 pair of small, rounded, pinkish bodies 

 lying on the external jugular veins. Their 

 secretion, which contains an organic com- 

 pound of iodine (thyroxine), has many 

 actions in the body, including bringing 



about the change from tadpole to adult (Fig. 284). The adrenal 

 bodies (so-called suprarenal glands) are small yellowish masses 

 lying on the ventral surface of the kidneys. They consist of two 

 kinds of tissue, which in the frog are mixed, while in man one, 

 the cortex, is a layer around the other, the medulla. 



The thymus is a small body which Hes behind and above the 

 angle of the jaw on each side. 



The pituitary body lies in the skull below the brain (see p. 387) 



Fig. 284. — Two individ- 

 uals of the same age 

 from the same batch 

 of frog's eggs. The one 

 on the right has 

 changed normally 

 into a frog ; that on 

 the left had the rudi- 

 ment of its thyroid 

 removed and has not 

 become adult but has 

 grown into a giant 

 tadpole. — From Hal- 

 dane and Huxley. 



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