12 HUMAN PHYSIOLOGY. 



by a decomposition of organic substances ; it is also produced within the tis- 

 sues as a result of chemical changes. Its function is unknown, though it is 

 asserted by Hoppe-Seyler that hydrogen unites with neutral oxygen, O 2 , in 

 the tissues, forming water and liberating oxygen in the nascent state, which 

 becomes the oxidizing agent. The process is represented in the following 

 equation : 



HH -f- O 2 -f n = H 2 O + On, 



in which n represents the oxidizable substance. 



Water is an essential constituent of all the tissues of the body, consti- 

 tuting about 70 per cent, of the entire body weight. It is introduced into 

 the body in the form of drink and as a constituent of all kinds of food. 

 The average quantity consumed daily is about four pints. While in the 

 body, water acts as a general solvent, gives pliability to various tissues, and 

 promotes the passage of inorganic and organic matters through animal 

 membranes. It also promotes chemical changes which are essential to 

 absorption and assimilation of food and the elimination of products of 

 waste. It is probable that water is also formed within the body by the 

 union of oxygen with the surplus hydrogen of the food. It is eliminated 

 by the skin, lungs and kidneys. 



Sodium Monde is present in all the solids and fluids of the body, with 

 the exception of enamel. It regulates osmotic action, holds the albuminous 

 principles of the blood in solution, and preserves the form and consistence 

 of blood corpuscles and the cellular elements of the tissues, by regulating 

 the amount of water entering into their composition. 



Calcium phosphate is the most abundant of all the inorganic principles 

 with the exception of water, and is present to a great extent in bone, teeth, 

 muscles and milk. It gives the requisite consistency and solidity to the 

 different tissues and organs. In the blood, it is held in solution by the 

 albuminous constituents. 



The Sodium and Potassium phosphates are present in most of the solids 

 and fluids, and give to them their alkaline reaction. They are chiefly 

 derived from the food. 



II. ORGANIC NON-NITROGENIZED PRINCIPLES. 

 The organic non-nitrogenized principles are derived mainly from the 

 vegetable world, but are also produced within the animal body. They are 

 divided into : 1st, the carbo-hydrates, comprising starch and sugar, bodies 

 in which the oxygen and hydrogen exfst in the proportion to form water, 

 the amount of carbon being variable; 2d, the fats, bodies having the same 



