24 PHYSIOLOGY FOR DENTAL STUDENTS. 



Lecithin is widely distributed in the animal body, and is VT\- 

 important in the metabolism and in the physical structure of the 

 cell. It consists chemically of glycerine, fatty acid, phosphoric 

 acid, and a nitrogenous base called cholin. 



Cholesterol is another widely distributed lipoid. It is not in 

 reality a fatty body, but rather resembles the terpencs. Lecithin 

 and cholesterol are abundant in brain tissue, in the envelopes of 

 erythrocytes, and in bile. 



The fats exist mainly as secondary constituents of the cell, 

 being deposited in very large amounts in certain of the connective 

 tissue cells of the body, in bone marrow and in the omental tis- 

 sues. Chemically, the tissue fats are of three kinds : olein, pal- 

 mitin, and stearin, each having a distinctive melting point. They 

 are compounds of the tri-valent alcohol, glycerine, and one of the 

 higher fatty acids, oleic, palmitic, or stearic acid. Besides those 

 that are present in the animal tissues, fats made up of glycerine 

 combined with various lower members of the fatty acid series 

 occur in such secretions- as milk. In order to understand the 

 influence which fats have on general metabolism, it is important 

 to remember that they differ from the carbohydrates in contain 

 ing a very low percentage of oxygen and a relatively high per- 

 centage of hydrogen and carbon. Thus, the empirical formula 

 of palmitin is C 51 H 98 8 or C 3 H g (C 16 H 31 2 ) 3 , that of dextrose 

 C 6 H 12 6 , and of protein C 72 H ll2 N 18 O aa S. 



THE CARBOHYDRATES are also mainly secondary cell constitu- 

 ents, although it is becoming more and more evident that they 

 are also necessary as primary constituents. In general they may 

 be defined chemically as consisting of the elements C, H, and 0, 

 the latter two being present in the molecule in the same propor- 

 tion as in water; thus, the formula for dextrose is C 6 H ]2 G . 



The basic carbohydrates are the simple sugars or monoxuc- 

 cliarides, such as grape sugar or dextrose. When two molecules 

 of monosaccharide become fused together with the elimination 

 of a molecule of water (thus giving the formula C^H^Ou), a 

 secondary sugar or disaccharide results. Cane sugar, lactose (or 

 milk sugar) and maltose (or malt sugar) are examples. If sev- 

 eral nonsaccharide molecules similarly fuse together, polysac- 



