THE THYROID BODY OR GLAND 217 



There are reasons for thinking that the glycogen, thus 

 deposited and stored up in the liver, is converted into 

 su>,'ar little by little as it is wanted, poured into the 

 hepatic vein, and thus distributed over the body. So that 

 we may regard this remarkable formation of glycogen in 

 the liver as an act by which the blood, when it is over- 

 rich in sugar, as after a meal, stores it up or deposits it in 

 the liver as glycogen ; and then, in the intervals between 

 meals, the liver deals out the stored-up material as sugar 

 back again in driblets to the blood. The loss to the blood 

 therefore, is temporary— no more a real loss than when a 

 man deposits at his banker's some money which he has 

 received until he has need to spend it. 



This story of glycogen, important in itself, is also use- 

 ful as indicating other possible effects of a similar nature 

 which the hepatic cells may bring about on the blood, as 

 it is passing in the meshes of the lobules of the liver from 

 the veinlets of the portal to the veinlets of the hepatic 

 vein. 



The contrast between the two types of function 

 exercised by the liver, the secretion of bile down the 

 bile duct and the secretion of sugar into the blood, was 

 emphasised by Claude Bernard, the celebrated French 

 physiologist, who discovered glycogen. The secretion 

 of bile he called an external secretion because it passed 

 away along a duct, the secretion of sugar into the blood 

 was termed an internal secretion. We shall have to con- 

 sider other examples of internal secretions and indeed the 

 secretory functions of glands which have no ducts must 

 be contined to the manufacture of such. 



17. The Thyroid Body or Gland —This organ consists 

 of two lobes, one lying each side of the trachea just below 

 the larynx and joined across the trachea by a connecting 

 strip of its own tissue. Each lobe is covered with a 

 capsule of connective tissue from which branches pass 

 inwards and divide the interior into rounded spaces or 

 alveoli. Each alveolus is lined by a layer of cubical cells 



