4 A MANUAL OF PHYSIOLOGY 



Fats and Lipoids. The fats are compounds of higher fatty acids 

 with glycerin (glycerin esters). The ordinary body-fat consists of 

 a mixture of three neutral fats (palmitin, stearin, and olein) which 

 differ both chemically and physically from each other e.g., in 

 melting-point and in the so-called iodine value, the number which 

 represents the amount of iodine taken up from a standard solution. 

 Olein melts at - 5 C., palmitin at 45 C., and stearin at a still higher 

 temperature. It is, therefore, the presence of olein which keeps the 

 body-fat liquid at the temperature of the body. The fats are soluble 

 in ether, in hot alcohol, and in many other liquids, but insoluble in 

 water. Besides the ordinary fats, the tissues and liquids of the body 

 contain lecithin (C 42 H 84 NPO 9 ), a fat-like compound which yields on de- 

 composition, in addition to glycerin, and a fatty acid, phosphoric acid 

 and a nitrogen-containing substance called cholin (p. 337). Lecithin, 

 though found in all cells, is especially abundant in nervous tissues. 

 It is associated with cholesterin and with other substances which, 

 like lecithin and cholesterin, are soluble in ether and similar solvents 

 of fat. For this reason these substances are often grouped together 

 as lipoids, although some of them are chemically quite different from 

 fat. Cholesterin, for instance, is an alcohol. Although usually 

 present only in small amount, the lipoids play a very important part 

 in the structure and in the economy of the cell. 



Structure of Living Matter The Cell. Protoplasm or living 

 substance, when examined in its most primitive, undifferentiated 

 condition in such cells as the amceba or the white blood-corpuscles, 

 appears on first view a homogeneous, structureless mass, except 

 for certain granules embedded in it, and consisting either of 

 products formed by its activity or of food materials. But even 

 here more careful study reveals a certain complexity of struc- 

 ture. At the very least, an external layer, or ectoplasm, can be 

 distinguished from the interior mass, or endoplasm. There is 

 reason to believe that even where no histological demonstra- 

 tion of an ectoplasmic layer or a definite envelope is 

 possible, the surface of the cell is physiologically different from 

 its interior. In many cells the protoplasm presents the appear- 

 ance of a honeycomb or network, with granules usually situated 

 at the nodes, and holding in its vesicles or meshes a fluid, perhaps 

 containing pabulum, from which the waste of the living frame- 

 work is made good, or material upon which it works, and which 

 it is its business to transform. Some observers, however, main- 

 tain that the network is an artificial appearance produced by the 

 precipitation of the colloid constituents of the protoplasm by the 

 fixing reagent, or even by the coagulative processes associated 

 with the act of dying, and that the unaltered living substance is 

 a homogeneous fluid or jelly. In certain respects it behaves like 

 a liquid, and in others like a solid, a peculiarity which is un- 

 doubtedly associated with its richness in colloids, as experiments 

 with such substances as gelatin and agar have shown. In 

 building up our typical cell we start with a piece of protoplasm. 

 Somewhere in the midst of this we find a body which, if not 



