818 PHYSIOLOGY OF DIGESTION AND SECRETION. 



liquids, and is held in solution in the bile by means of the bile-acids. 

 Provisionally we may regard it, so far as its occurrence in the bile is 

 concerned, as a waste product of cellular disintegration which is 

 eliminated in the feces. It is excreted also through the skin, in 

 the sebaceous and sweat secretions, and in the milk. 



Lecithin, Fats, and Nucleo-albumin. Lecithin, C^Hgo- 

 NPO 9 , is a compound of glycerophosphoric acid with fatty acid 

 radicals (stearic, oleic, or palmitic) and a nitrogenous base, cholin 

 (see p. 78). When hydrolyzed by boiling with alkali it splits 

 up into these three substances. It is found generally as such, 

 or in combination, in all cells, and evidently plays some as yet 

 unknown part in cell metabolism. It occurs in largest quantity 

 in the white matter of the nervous system. In the liver it occurs 

 to a considerable extent both as lecithin and in a more complex 

 combination with a carbohydrate residue, a compound designated 

 as jecorin. So far as it is found in the bile, it represents possibly 

 a waste product derived from the liver or from the body at large, 

 although it is possible that it may undergo hydrolysis in the intes- 

 tine and be absorbed in the form of its split products. Little is 

 known of its precise physiological significance. According to 

 Hewlett and others it may serve to activate the lipase of the pan- 

 creatic secretion. 



The special importance, if any, of the small proportion of fats 

 and fatty acids in the bile is unknown. The ropy, mucilaginous 

 character of bile is due to the presence of a body formed in the bile- 

 ducts and gall-bladder. This substance was formerly designated 

 as mucin, but it is now known that in ox bile at least it is not a true 

 mucin, but a nucleo-albumin (see appendix). Hammarsten reports 

 that in human bile some true mucin is found. Outside the fact that 

 it makes the bile viscous, this constituent is not known to possess any 

 especial physiological significance. 



The Secretion of the Bile. Numerous experiments have been 

 made to ascertain whether or not the secretion of bile is controlled 

 by a special set of secretory fibers. The secretion itself is continuous, 

 but varies in amount under different conditions. These conditions 

 may be controlled experimentally in part. It has been shown, for 

 example, that stimulation of the spinal cord or splanchnic nerve 

 diminishes the flow of bile, while section of the splanchnic branches 

 may cause an increased flow. These and similar actions are ex- 

 plained, however, by their effect on the blood-flow through the liver. 

 The splanchnics carry vasomotor nerves to the liver, and section or 

 stimulation of these nerves will therefore alter the circulation in the 

 organ. Since the secretion increases when the blood-flow is increased 

 and vice versa, it is believed that in this case no special secretory 

 nerve fibers exist. The metabolic processes in the liver cells which 



