4 INTRODUCTION 



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 phospha- 

 tides, a group of compounds which stand in close relation to the fats, 

 but differ in containing phosphoric acid and nitrogenous bases. The 

 most important representative of this group is lecithin ^H^NPC^), 

 a fat-like compound which yields on decomposition, in addition to 

 glycerin and a fatty acid, phosphoric acid and a nitrogen-containing 

 substance called cholin (p. 360). Lecithin, though found in all cells, is 

 especially abundant in nervous tissues. It is associated with choles- 

 terin 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 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.* -Bioplasm is the name 

 given to the living matter of cells. The portion of the bioplasm 

 differentiated as the nucleus is distinguished by the term karyo- 

 plasm, and the portion outside the nucleus by the term protoplasm 

 or cytoplasm. Protoplasm, when examined in its most primitive 

 undifferentiated condition in such cells as the amoeba 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 dis- 

 tinguished from the interior mass, or endoplasm. There is reason 

 to believe that even where no histological demonstration 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 appearance of a honeycomb or net- 

 work, 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 framework is made good, or material 

 upon which it works, and which it is its business to transform. 

 Some observers, however, maintain 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. It is known that 



* Space permits only the slightest sketch of this subject here. For de- 

 tailed information the student is referred to textbooks of histology. 



