SECRETION. 245 



discussion and investigation : first, the relationship existing between the liver- 

 cells and the lining epithelium of the bile-duets ; second, the presence or ab- 

 sence of a distinct membranous wall for the bile-capillaries. Different opin- 

 ions are still held upon these points, but the balance of evidence seems to show 

 that the bile-capillaries have no proper wall. They are simply minute tubular 

 spaces penetrating between the liver-cells and corresponding to the alveolar lu- 

 men in other glands. Where the capillaries join the interlobular ducts the liver- 

 cells pass gradually or abruptly, according to the class of vertebrates examined, 

 into the lining epithelium of the ducts. From this standpoint, then, the liver- 

 cells are homologous to the secreting cells of other glands in their relations to 

 the general lining epithelium. Several observers (MaCallum, 1 Berkley, 2 and 

 Korolkow 3 ) have claimed that they are able to trace nerve-fibres to the 

 liver-cells, thus furnishing histological evidence that the complex processes oc- 

 curring in these cells are under the regulating control of the central nervous 

 system. According to the latest observers (Berkeley, Korolkow) the terminal 

 nerve-fibrils end between the liver-cells, but do not actually penetrate the sub- 

 stance of the cells, as was described in some earlier papers. If these observa- 

 tions prove to be entirely correct they would demonstrate the direct effect of 

 the nervous system on some at least of the manifold activities of the liver- 

 cells. So far as the formation of the bile is concerned we have no satisfactory 

 phvsiological evidence that it is under the control of the nervous system. 



Composition of the Secretion. — The bile is a colored secretion. In 

 most carnivorous animals it is golden red, while in the herbivora it is green, 

 the difference depending on the character and quantity of the pigments. In 

 man the bile is usually stated to follow the carnivorous type, showing a red- 

 dish or brownish color, although in some cases apparently the green predomi- 

 nates. The characteristic constituents of the bileare the pigments, bilirubin in 

 carnivorous bile and bilivcrdin in herbivorous bile, and the bile acids or bile- 

 salts, the sodium salts of glycocholic or taurocholic acid, the relative proportions 

 of the two acids varying in different animals. In addition there is present a 

 considerable quantity of a mucoid nucleo-albumin, a constituent which is QOl 

 formed in the liver-cells, but is added to the secretion by the mucous membrane 

 of the bile-ducts and gall-bladder ; and small quantities of cholesterin, lecithin, 

 fats, and soaps. The inorganic constituents comprise the usual salts — chlorides, 

 phosphates, carbonates and sulphates of the alkalies or alkaline earths. Iron 

 is found in small quantities, combined probably as a phosphate. The secre- 

 tion contains also a considerable though variable quantity <>f CO a gas, held in 

 such loose combination that it can be extracted with the gas-pump without the 

 addition of acid. The presence of this constituent serves a- an indication of 

 the extensive metabolic changes occurring in the liver-cells. Quantitative 

 analyses of the bile show that it varies greatly in composition even in the same 

 species of animal. Examples of this variability are given in the analyses 



1 MaCallum : Quarterly Journal of the Microscopical Sciences, 1887, vol, x.wii. p. 189. 

 ■■* Berkley : Anatomischer Aruseiger, 1893, Bd. viii. 8. 769. Korolkow: Ibid., 8. 750. 



