39 THE CHEMICAL COMPOSITION OF THE BODY. [CH. xxv. 



Tt is readily soluble in water, but insoluble in alcohol and 

 ether. It is gummy and amorphous. It does not give Trom- 

 mer's test, nor does it ferment with yeast. It is dextro-rotatory. 

 By hydrating agencies it is converted into glucose. 



Glycogen, or animal starch, is found in liver, muscle, and 

 white blood-corpuscles. It is also abundant in all embryonic 

 tissues. 



Glycogen is a white tasteless powder, soluble in water, but it 

 forms, like starch, an opalescent solution. It is insoluble in 

 alcohol and ether. It is dextro-rotatory. With Trommer's test 

 it gives a blue solution, but no reduction occurs on boiling. 



With iodine it gives a reddish or port -wine colour, very similar 

 to that given by erythro-dextrin. Dextrin may be distinguished 

 from glycogen by (i) the fact that it gives a clear, not an opales- 

 cent, solution with water; and (2) it is not precipitated by basic 

 lead acetate as glycogen is. It is, however, precipitated by basic 

 lead acetate and ammonia. (3) Glycogen is precipitated by 55 

 per cent, of alcohol ; the dextrins require 85 per cent, or more. 



Cellulose. This is the colourless material of which the cell- 

 walls and woody fibres of plants are composed. By treatment 

 with strong mineral acids it is, like starch, converted into glu- 

 cose, but with much greater difficulty. The various digestive 

 ferments have little or no action on cellulose ; hence the necessity 

 of boiling starch before it is taken as food. Boiling bursts the 

 cellulose envelopes of the starch grains, and so allows the digestive 

 juices to get at the starch proper. 



Cellulose is found in a few animals, as in the test or outer 

 investment of the Tunicates. 



[Inosite, or muscle sugar, is found in muscle, kidney, liver, and 

 other parts of the body in small quantities. It is also largely 

 found in the vegetable kingdom. It is crystallisable, and has 

 the same formula as the glucoses. It is, however, not a sugar, 

 and careful analysis has shown that it really belongs to the 

 aromatic series.] 



The Fats. 



Fat is found in small quantities in many animal tissues. It 

 is, however, found in large quantities in three situations, viz., 

 marrow, adipose tissue, and milk. 



The contents of the fat cells of adipose tissue are fluid during 

 life, the normal temperature of the body (36 C., or 99 F.) being 

 considerably above the melting-point (25 C.) of the mixture of 

 the fats found there. These fats are three in number, and are 

 called palmitin, stearin, and olein. They differ from one another 



