302 PRACTICAL PHYSIOLOGY 



acids in fats. Its determination involves the use of carefully 

 standardised solutions, and is too complicated for description here. 



VII. The Reichert-Meissl value indicates the amount of volatile 

 soluble fatty acids present. It is of great value in testing the purity of 

 butter, because this contains a considerable proportion of such acids, 

 whereas the cheaper fats, which are sometimes used as substitutes for 

 butter, do not contain much of them. 



EXPERIMENT. 5 gms. melted fat is saponified with alcoholic potash, 

 the alcohol evaporated, and the resulting soap dissolved in water 

 acidified with sulphuric acid, and distilled. The distillate, which 

 contains the volatile acids, is collected in a flask and titrated with 



N 



JQ NaOH, the result being expressed as the number of c.c. of decinormaJ 



acid contained in the distillate from five gms. of fatty substance. 



VIII. The Hehner value indicates the amount of non-volatile and 

 insoluble fatty acids (and unsaponifiable matter) present in the fat. 



IX. The Acetyl value gives the amount of KOH which is required to 

 combine with the acetic acid in 1 gm. of fat previously acetylated, 

 i.e. the hydroxyl value. 



CHAPTER XX 

 THE FAECES 



These are composed of : 



(1) Substances which have escaped digestion, e.g. muscle fibres, 

 elastic tissue, cellulose, fat, etc. 



(2) Remains of gastro -intestinal secretions. 



(3) Products of digestion by ferments or bacteria. 



(4) Micro-organisms. These often form quite a marked proportion of 

 the faecal mass. It has been said that as much as a half of the total 

 nitrogen in the faeces may be of bacterial origin. 



Amount. The daily faecal excretion of an adult male upon an ordinary 

 mixed diet will average from 100 to 170 gms. with a solid content of 

 between 25 and 45 gms. The amount of faeces passed when the 

 subject is on a vegetable diet is much greater, may even amount to 

 400 gms. or more. The water content varies within fairly wide 

 limits. The colour depends largely upon the nature of the diet, a meat 

 diet, for instance, giving a dark brown colour and a milk diet a light- 

 coloured stool. The colour may be altered by the taking of drugs 

 such as iron, calomel or bismuth. 



Reaction. Normal stools have, as a general rule, a neutral reaction 

 although slightly alkaline or slightly acid faeces are frequently met 

 with, the alkaline reaction being more common than the acid one. 



Nitrogenous Substances. Faeces always contains a certain amount of 

 nitrogenous matter, from 0-5 to 2 gms. per diem. Most of this probably 

 comes from waste material from the various digestive secretions, 

 micro-organisms, etc. The proteins in vegetable food-stuffs give rise 

 to a greater amount of nitrogen in the faeces than do proteins of animal 

 origin, since digestion is often less perfect. 



Lipoid Substances. Faeces always contain under perfectly normal 

 conditions a fair amount of lipoid material, n.esutva.1 fats, free and 



