COMPOSITION OF THE BODY. 45 



come under the nitrogenous group ; " even the non- 

 cellular liquids passing out into the alimentary canal 

 at various points which have so great an action in 

 preparing the food in different ways are not only 

 nitrogenous, but the constancy of this implies the 

 necessity of the nitrogen, in order that these actions 

 shall be performed." (Parkes.) White of egg is a familiar 

 example of this group. These nitrogenous substances 

 consist of carbon of which charcoal is a well-known 

 form hydrogen and oxygen the two constituents of 

 water combined in various propotions with nitrogen, 

 and, in the case of albuminous substances, with a 

 small amount of sulphur. 



Both the fats and saccharine matters are composed 

 of certain combinations of carbon, hydrogen and 

 oxygen. In the latter, the two last-mentioned elements 

 are united in the proportion that forms water, so that 

 sugar and starch may be regarded as a combination 

 of carbon with that fluid ; but in the former, there is 

 a smaller amount of oxygen. The fat, which is 

 deposited as a layer immediately under the skin, serves 

 to maintain the internal temperature of the body, by 

 the fact of its being a bad conductor of heat. Fat 

 also acts as a natural elastic cushion to various parts 

 of the system, as, for instance, at the back of the eye- 

 ball, above the horny frog, and around the joints. 



Dr. Carpenter remarks, that "the muscular, nervous, 

 and glandular tissues are not composed of albuminous 

 substances alone; they contain, as an essential con- 

 stituent of their structure, a certain portion of fat, 

 without which their composition would be imperfect, 

 and the performance of their functions impossible." 



