748 PHYSIOLOGY OF DIGESTION AND SECRETION. 



body. According to Naunyn, however, the cholesterin is not secreted 

 by the liver cells proper, but is added to the secretion while in the bile 

 passages the gall-ducts and gall-bladder. That it is an excretion 

 is indicated by the fact that it is eliminated unchanged in the feces. 

 Cholesterin is insoluble in water or in dilute saline liquids, and is held 

 in solution in the bile by means of the bile acids. We must regard 

 it as a waste product of cell life, formed probably in minute quantities, 

 and excreted mainly through the liver. It is partly eliminated 

 through the skin, in the sebaceous and sweat secretions, and in the 

 milk. 



Lecithin, Fats, and Nucleo-albumin. Lecithin, C 44 H 90 NPO 9 

 is a compound of glycerophosphoric acid with fatty acid radicals 

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

 hydrolyzed by boiling with alkali it splits up into these three sub- 

 stances. 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. Little is known of its precise physiological sig- 

 nificance. 



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 



