THE ORGANS OF EXCRETION 263 



though the general blood pressure is not raised, it does 

 appear to affect directly the action of the kidneys and to 

 increase the excretion. 



385. The Kidneys and the Nervous System. The state of 

 the central nervous system greatly affects the activity of 

 the kidneys. This may be by the passage of emotional 

 impulses, originating in the brain, along vasodilator fibers 

 to the kidneys. The blood vessels being thus dilated, the 

 activity of the glands would be stimulated. Very large 

 quantities of almost pure water are sometimes thus elimi- 

 nated under emotional excitement. 



386. Excretion by the Alimentary Canal. Portions of the 

 food taken into the stomach are unfit to enter into the 

 structure of the tissues, and are expelled unchanged 

 through the intestines. A small amount of true excretion 

 also takes place there, by which used-up matter is removed. 



387. Autointoxication, or Self-poisoning. Even under 

 normal conditions man's organism is " a receptacle and a 

 laboratory of poisons." They are taken in with the food 

 in the form of those minute bodies which cause putrefac- 

 tion. The process of katabolism ( 283) is a manufacture 

 of poisons, and many of the secretions, such as the saliva 

 and the bile, are poisonous. A large number of poisonous 

 substances are formed in the intestines, and several forms 

 of deadly poison have been discovered in the urine. All 

 of the poisons made by the tissues, and some of those 

 manufactured in the digestive tube, are poured into the 

 blood, so that even normal blood contains poisons, though 

 an excessive amount of the same poisons threatens health 

 and life. Many diseases are now understood to result from 

 the self-poisoning due to disordered nutrition, along with 

 the opportunities for infection from specific germs which 

 are always present; and reabsorption of excrementitious 



