296 DIGESTION OF FATS. [BOOK n. 



thoracic duct after a meal ; but there is no proof that any large 

 quantity of fat is introduced in this form into the circulation. On 

 the other hand, the presence of neutral fats, both in portal blood, 

 and especially in the lacteals, is a conspicuous result of the diges- 

 tion of fatty matters ; and in all probability saponification in the 

 intestine is a subsidiary process, intended rather to facilitate the 

 emulsion of neutral fats than to introduce soaps as such into the 

 blood. For the presence of soluble soaps favours the emulsion of 

 neutral fats. Thus a rancid fat, i.e. a fat containing a certain 

 amount of free fatty acid, forms an emulsion with an alkaline fluid 

 more readily than does a neutral fat. A drop of rancid oil let fall 

 on the surface of an alkaline fluid, such as a solution of sodium 

 carbonate of suitable strength, rapidly forms a broad ring of emul- 

 sion, and that even without the least agitation. As saponification 

 takes place at the junction of the oil and alkaline fluid currents 

 are set up, by which globules of oil are detached from the main 

 drop and driven out in a centrifugal direction. The intensity of 

 the currents and the consequent amount of emulsion depend on 

 the concentration of the alkaline medium and on the solubility of 

 the soaps which are formed ; hence some fats such as cod-liver oil 

 are much more easily emulsionized in this way than others. Now 

 the bile and pancreatic juice supply just such conditions as the 

 above for emulsionizing fats : they both together afford an alkaline 

 medium, the pancreatic juice gives rise to an adequate amount of 

 free fatty acid, and the bile in addition brings into solution the 

 soaps as they are formed. So that we may speak of the emulsion 

 of fats in the small intestine as being carried on by the bile and 

 pancreatic juice acting in conjunction ; and as a matter of fact the 

 bile and pancreatic juice do largely emulsify the contents of the 

 small intestine, so that the greyish turbid chyme is changed into a 

 creamy-looking fluid, which has been sometimes called chyle. It 

 is advisable however to reserve this name for the contents of the 

 lacteals. 



This mutual help of bile and pancreatic juice in producing an 

 emulsion, explains to a certain extent the controversy which long 

 existed between those who maintained that the bile and those who 

 maintained that the pancreatic juice was necessary for the diges- 

 tion and absorption of fatty food. That the pancreatic juice does 

 produce in the intestine such a change as favours the transference 

 of neutral fats from the intestine into the lacteals, is shewn by the 

 fact that in diseases affecting the pancreas, much fatty food 

 frequently passes through the intestine undigested, and great 

 wasting ensues ; but it cannot be maintained that the pancreatic 

 juice is the sole agent in this matter, since in animals in which the 

 pancreatio ducts have been successfully ligatured chyle is still 

 found in the lacteals. On the other hand, that the bile is of use in 

 the digestion of fat is shewn by the prevalence of fatty stools in 

 cases of obstruction of the bile-ducts; and though the operation of 



