92 DIGESTION 



the working active cells with nutriment which enables them 

 to carry on their several functions, and — in growing tissues — 

 to increase in size and to multiply the number of their cells. 



Hormones and Vitamins 



When the food taken by the mouth reaches the stomach 

 and intestine, the digestive jmces are poured out to meet it. 

 The action is started by means of hormones, which in some 

 way stimulate the secretory cells. The name hormone is 

 derived from opfidco, "I arouse to activity." These substances 

 act as chemical messengers : when the food mixed ^vith gastric 

 juice enters the small intestine, a substance called "secretin" 

 is set free and passes by the blood stream to the pancreas. 

 Its action on the pancreatic cells is to cause the pouring out 

 of the pancreatic juice into the intestine. It has been proved 

 that the agent which causes the liberation of "secretin" is 

 the acid of the gastric juice. An acid extract of intestine, 

 when injected into an animal, will cause a copious flow of 

 pancreatic juice even though the animal has taken no food. 

 Hormones are j^roduced by numerous glands in the body and 

 exercise great influence on many processes, such as meta- 

 bolism, respiration, growth, and the sexual characters. In 

 only two instances, namely those of adrenaline and thyroxin, 

 have hormones been isolated in a pure state and their chemical 

 composition determined. They have both a com23aratively 

 simple structure, and there is reason to believe that other 

 hormones are not of great complexity. Many are not destroyed 

 by boiling, but the fact that they are present in such minute 

 quantity and are adsorbed on to other substances renders their 

 isolation exceedingly difficult. 



The hormones of the body are arranged under a complex 

 system. There appears to be co-operation and antagonism 

 between different groups. The over production of one or the 

 deficiencj?" of another will upset the balance and cause far- 

 reaching changes. Many disorders are due to a disturbed 

 hormone balance. What knowledge we have of them has been 

 gained by the removal of some of the glands from animals, 

 or injection of extracts of glands. The symptoms produced can 



