18 STUDIES IN PHYSIOLOGY 



Nitrogenous Substances. The most important substances 

 in the living body are the ni-trog'e-nous compounds. They 

 are all characterized, as the name implies, by the presence of 

 the element nitrogen (symbol N). Some of these com- 

 pounds which are present in all living substance are known 

 as pro'te-ids or albuminous substances. Without exception, 

 proteids consist chiefly of the elements carbon, hydrogen, 

 oxygen, nitrogen, and sulphur, and often other elements are 

 present in their composition. They are the most complex 

 substances in the body. 1 



From the carbon of the proteids also there is formed by 

 oxidation the waste gas carbon dioxid, which, as we have 

 demonstrated, is thrown off from the lungs. Another im- 

 portant waste compound that comes from the oxidation of 

 proteids is a substance known as u're-a. Urea contains 

 most of the nitrogen that cannot be further used by the 

 body. It is taken from the blood by the kidneys and forms 

 the principal solid constituent dissolved in the urine. 



The summary on the following page contains in brief the 

 principal facts we have learned in regard to the chemical 

 composition of the body. 



1 The composition, for example, of the proteid hem-o-glo'bin (Greek 

 hai'ma= blood) which gives the red color to the blood, is said by one 

 chemist to be CeooHgeoOngN^SsFe (Fe being the symbol for iron). 

 This formula means that a molecule of the compound hemoglobin 

 consists of over eighteen hundred atoms, six hundred of which are 

 carbon, nine hundred and sixty, hydrogen, etc. 



