56 PHYSIOLOGY 



tension between the neutral fat and the water. Like many other colloidal 

 solutions, a soap solution presents the phenomenon of surface aggregation, 

 i.e. the concentration of the soap at the surface is increased to such an extent 

 as to form practically a solid pellicle of molecular dimensions on the surface of 

 the fluid. The same pellicle formation occurs at the surface of every oil 

 globule, so that on shaking up rancid oil with dilute sodium carbonate, the 

 whole of the oil is broken up into fine droplets which show no tendency to 

 run together again, and remain in suspension in the water. The suspension 

 of fine oil droplets, which has the appearance of milk, is spoken of as an 

 emulsion. It can be at once destroyed by adding acid. This decomposes 

 the soap, setting free the fatty acids, which are insoluble in the water. The 

 pellicle around each globule is destroyed, and the globules run together as 

 neutral oil would in pure water. 



In order to characterise any given animal fat or mixture of fats the following reactions 

 are made use of : 



(1) The 'acid number ' of the fat, i.e. its content in free fatty acids, is determined 



N 

 by titrating it in ethyl alcohol solution with ^ alcoholic solution of potash, using phenol - 



phthalein as an indicator. 



(2) The ' saponification number.' This represents the number of milligrammes 

 of potassium hydrate necessary to saponify completely one gramme ot fat. 



(3) The percentage of volatile fatty acids is determined by saponifying the fat, 

 then treating it with a mineral acid to set free the fatty acids and distilling over the 

 volatile acids in a current of steam. 



(4) The iodine number is the amount of iodine which is taken up by a given weight 

 of fat. It is a measure of the amount of unsaturated fatty acid present, i.e. in ordinary 

 fat, oleic acid. 



Besides the glycerides, a certain number of substances occur in the body 

 derived, not from a combination of fatty acids with glycerol, but from a 

 formation of esters of the fatty acids and other alcohols, e.g. cholesterol or 

 cetyl alcohol. Thus, spermaceti is a mixture of cetyl palmitate with small 

 quantities of other fats. The fatty secretion of the sebaceous glands in 

 man and the higher animals, which furnishes the natural oil of hair, wool, 

 and feathers, consists, with small traces of glycerides of cholesterol esters. 

 Lanoline, which is purified wool fat, consists almost entirely of cholesteryl 

 stearate and palmitate. These cholesterol fats are attacked with extreme 

 difficulty by ferments or micro-organisms. It is probably on this account 

 that they are manufactured in the body for protective purposes. So far as 

 we know, when once formed, they are incapable of further transformation in 

 the body. They are not appreciably altered by the digestive ferments of the 

 alimentary canal, and the cholesterol is said to pass through the latter 

 unaltered.* Cholesterol is also found in combination with fatty acids in 

 every living cell. Whenever protoplasmic structures are extracted with 

 boiling ether, a certain amount of cholesterol is present with the fats which 

 are so extracted. In view of the great stability of this substance when 

 exposed to the ordinary mechanisms of chemical change in the body, it seems 

 * According to Gardner, cholesterol may be absorbed from the intestine. 



