Because the _liver_ secretes a fluid to assist in the digestion of food. The liver is a gland--a similar organ to the glands of the mouth--and it forms _bile_ in the same manner that they form the salivary juice. Only the liver is a _much larger gland_, and a much greater quantity of blood passes through it. The liver pours its secretion into the biliary duct (Fig. 49) to mix with the grey cream as it passes onward, and to further dissolve it. But when the stomach is excited by food which it cannot dissolve, and when the owner of the stomach, disregarding its remonstrances, will persist in over-eating, or in eating things that disagree with the system, then _the liver and the stomach sympathise_, and the muscular threads, or hands, that prevail all through the alimentary organs, instead of moving _onward_, move backward, and _throw some bile into the stomach_ to assist to dissolve and remove the excessive or improper food. CHAPTER XLII. 883. _Why does some portion of the food we eat nourish the system, while other portions are useless?_ Because most food contains some particles that are indigestible, or that, if digested, are innutritious, and not necessary for the system. The _liver_ is the organ by whose secretion the _useful is separated from the useless_; for when the bile enters through the duct (Fig. 49) and mixes with the grey cream coming from the stomach, it remains no longer a grey cream, but turns into a mass coloured by bile, having upon its surface _little globules of milk_, small, but very white. Those minute globules of milk (_chyle_) are the nutritious particles derived from the food; the other portion, coloured with bile, is the useless residue, or rather the _bulk from which the nutrition has been extracted_. [Verse: "God hath made of one blood all nations of men for to dwell on all the face of the earth."--ACTS XVII.] 884. _Why does the milky, or nutritious matter, separate from the innutritious, upon admixture with bile?_ Because the bile contains an oily matter which _repels_ the watery _milk of nutrition_. The _pancreatic juice_ also enters through the same duct with the bile. But its precise use is not understood. It is a fluid much like the salivary secretion of the glands of the mouth. [Illustration: Fig. 50.--GREAT VESSELS OF THE CIRCULATION, AND THE DUCT WHICH CONVEYS NUTRITIVE MATTER TO THE BLOOD.] A B. _Jugular veins_ which return blood from the head to the heart. C. The _superior venæ cava_, or trunk vein, which pours the blood returned from the upper part of the system into the heart. There is a similar large vessel which meets this one and brings back blood from the lower part of the body, and they both pour the blood into the right side of the heart. D E. The branches of the _venous system_ which bring back the blood from the arms.