26 INTEODTJCTION. 



proximate principles is of organic origin, definite chemical 

 composition, and crystallizable. With the exception of the 

 salts peculiar to the bile, which will be considered when we 

 come to treat of that fluid, pneumic acid, and one or two 

 unimportant principles, they are distinguished by being com- 

 posed of three elements, Carbon, Hydrogen, and Oxygen. 

 As they thus contain hydrogen and carbon, to the exclusion 

 of all other elements, except the almost universal principle, 

 oxygen, they are frequently spoken of as Hydro-carbons. 

 They are distinguished from other organic substances by the 

 absence of nitrogen, which has given them the name of Non- 

 nitrogenized or Non-azotized substances. They are intro- 

 duced into the body as food, and are manufactured in the 

 economy by special organs ; but, unlike principles of the first 

 class, with the exception of sugar and fat, which are dis- 

 charged in the milk during lactation, are never discharged 

 from the body in health. The principles of this class play 

 an important part in development and nutrition. One of 

 them, sugar, appears very early in foetal life, formed first by 

 the placenta, and afterwards by the liver ; its formation by 

 the latter organ continuing during life. Fat is a necessary 

 element of food, and is also formed in the interior of the 

 body. The exact influence which these substances have on 

 development and nutrition is not known, but experiments 

 and observation have shown that this influence is important. 

 Many physiologists are of the opinion that principles of this 

 class undergo direct oxidation or combustion in the lungs, and 

 have the exclusive office of keeping up the animal tempera- 

 ture. At one time, indeed, they were generally spoken of as 

 calorific elements ; but in the present condition of science this 

 exclusive view is not tenable ; and we shall see, when treating 

 of the subject of animal heat, that its production cannot be 

 referred entirely to combustion of the hydro-carbons. The 

 sugars and fats, lactic acid and the lactates, pneumic acid and 

 the pneumates, the fatty acids and their combinations, consti- 

 tute the most important principles of this class. 



