Chemical Constituents of the Animal Body. 189 



chemical constants, (a) The saponiftcation value gives an 

 idea of the molecular weight of the acids, (b) The iodine 

 value gives some measure of the degree of unsaturation of 

 the acids, (c) The Reichert-Meissl value indicates roughly 

 the amounts of acid (of low molecular weight) which are 

 readily volatile, (d) The acetyl value gives a measure of 

 the hydroxyl groups in the acids. These values are more 

 or less empirical, and they are determined under certain 

 standard conditions of experiment. The fats are the chief 

 constituent of the animal tissues which can be removed by 

 organic solvents. 



ADDENDUM TO PART I. THE LIPOIDS. 



The term " lipoid," as already stated, is a somewhat 

 indefinite one. It may be held to include those substances 

 which are not glycerides which can be extracted from 

 animal tissues or fluids. Similar substances can also be 

 obtained from plant tissues ; the latter yield, however, a 

 number of other products soluble in organic solvents which 

 are neither fats nor lipoids. 



Although lipoids play a very important part in the 

 general economy of the cell, little is at present known 

 either of their chemical properties or of their true biological 

 significance. It is not, therefore, possible to deal with them 

 in any detail. In certain tissues, such as in those of the 

 central nervous system, they are found in relatively large 

 quantities. Owing to their physical properties, they are 

 not readily obtained in a pure state. The difficulties of 

 investigation are also complicated by the fact that in the 

 majority of animal tissues and fluids, such as the blood 

 and the liver, they only exist in small amounts. 



The chief lipoids are the alcohol cholesterol, which 

 exists in small quantities in most tissuee of vertebrate 

 animals, and the chemically allied phytosterols of plant 



