8 Feb.. 1907.] I\lciiiciiis of Aiiiiual I'Jivsiology. ' 87 



proper vields onlv dextrose on treatment with acids, the hemicelluloses give 

 rise to mixtures of various simple sugars. Thev are found in the outer 

 covering of cereal seeds and in the reserve cellulose of plants. 



The substance lignin may he mentioned here. It is found in wood and 

 in coarse woodv fibre, and is a still more resistant substance than cellu- 

 lose. Its chemical constitution has not been made out, and its chief physio- 

 logical importance is that it passes through the animal bowel unchanged. 



The cellulose group is quite unrepresented in mammalian tissues. 



5. Pentosans. — ^These polysaccharides are compounded of pentoses, as 

 treatment with an acid testifies. They are found chiefly in vegetable gums, 

 but are also present in woody fibre. Pentosans are not found in the animal 

 body, and their food value is very low. 



Fats and Lipoid. 



Under the term fats we include not only ordinary animal fats but also 

 the animal and vegetable oils, for these latter are simplv fats that are 

 linuid at ordinarv temperatures. The term, however, does not include the 

 mineral oils such as kerosene and paraffin oil, nor the " essential oils" of 

 plants, which belong to totalh- different chemical groups and can neither 

 act as foods nor take any part in forming the structures of the animal body. 

 As common characters of fats which are well known we might mention that 

 they are insoluble in water, and refuse to mix with it, and, as thev are 

 lighter weight for weight, they float on the surface of water. Thev boil 

 at a temperature which is generally higher than the boiling-point of water, 

 and thev are readilv inflammable. 



Qualities not so generally known are the following : Thev are soluble 

 in ether, chloroform and benzine. They are soluble in hot alcohol, but, as 

 J rule, separate out in solid form on cooling. When liquid and when 

 shaken with water containing even a small quantitv of soap- thev break up 

 into a multitude of droplets of microscopic size. These droplets, on account 

 of their minuteness, rise to the surface so slowlv that the fat may exist for 

 a considerable time evenly distributed through the mixture — such a mixture 

 being termed an emulsioft. This formation of an emulsion in the presence 

 of soap not only accounts for the detergent or cleansing action of this sub- 

 stance, but is, as we shall see, of the utmost importance in the digestion of 

 fats. 



Before we can understand the chemistry of fats mention must be made 

 of the FATTY ACIDS. These acids contain carbon, hydrogen and oxygen, 

 the carbon and hydrogen being greatly in excess of the oxvgen. A large 

 number are known to the chemist, and are classified in a regular series, the 

 lower members of which (such as acetic acid) being liquid at ordinarv 

 temperatures and soluble in water, the higher members (such as stearic 

 acid, present in certain candles and quite erroneously termed stearin) being 

 solid at ordinary temperatures and insoluble in water. Between these 

 extremes are intermediate forms, but the majority have solubilities like the 

 fats; are, namely, insoluble in water but soluble in ether or hot alcohol. 

 Manv of the fattv acids possess a characteristic and often a verv unpleasant 

 smell. 



Now, when a fatt\- acid forms a chemical compound with an alkali the 

 result of the union is a soap; but when the fattv acid forms a chemical 

 compound with glycerine the result is a fat. That fats are so compounded 

 is shown by man\; facts and experiments. Fats, when subjected to the 



