CHAPTER XI 



THE DUCTLESS GLANDS AND THE ORGANS OF 

 INTERNAL SECRETION 



The term 'Internal Secretion' was first used by Claude 

 Bernard Avho applied it to describe the glycogenic function of 

 the liver. This gland, as has already been recorded, stores up 

 carbohydrate as glycogen and secretes it into the blood, as 

 required, in the form of sugar. The liver, then, in addition to 

 being an externally secreting gland (that is to say, a gland which 

 elaborates substances (in this case bile) which are discharged 

 outwards through a duct) is also an internally secreting gland. 

 In one sense all the organs and tissues of the body are internally 

 secreting organs since the substances which pass out from them 

 into the circulating blood are different from the substances which 

 pass in. But the term 'internal secretion' is usually restricted 

 to those cases where the substances elaborated by the organs in 

 question have a precise function and act upon other organs or 

 tissues in the body in a definite way. The internally secreting 

 organs therefore are those which produce ' hormones ' or chemical 

 excitants which after being carried throughout the body in the 

 blood stream promote the secretion of particular glands or the 

 growth of particular tissues for which they have a specific action. 

 Thus, the liver although it is in a very literal sense an organ of 

 internal secretion is not ordinarily included in that categorj^ 

 since it does not elaborate any bodies included under the term 

 hormone. 



We have already dealt with typical hormones in describing 

 the mechanisms of pancreatic and gastric secretion, the chemical 

 excitants in these cases being secreted by the wall of the duo- 

 denum and the pyloric end of the stomach respectively. We 

 have seen also that the waste product carbon dioxide may be 

 regarded as of the nature of a hormone, since when its tension 

 in the blood reaches a certain point, it has a specific exciting 

 action upon the respiratory centre in the medulla, thereby 

 quickening the respiratory movements. It is possible that the 



