THE LIVER. 537 



161. Excretory ducts of the liver. The biliary duct and its branches 

 accompany the vena portce and hepatic artery, so that on one side of a 

 portal branch there is always a much smaller biliary duct and artery, 

 which are included with it in a sheath of connective tissue, the so-called 

 capsule of Glisson. The hepatic ducts ramify, in Man, with the vena 

 portce and may be dissected out for a long distance ; and with the mi- 

 croscope they may, in fresh and injected livers, be traced as far as the 

 lobules. Before reaching the latter, they either do not anastomose at 

 all or very sparingly ; the ductus interlobulares, however, as they have 

 been termed, appear to be continuous with each other and thus to invest 

 the hepatic islets. From these ducts of 1-90-1-120 of a line branches 

 of 1-100-1-120 of a line proceed, in no very great numbers, to the 

 hepatic islets and become continuous with the hepatic network in the 

 mode above described. Perhaps the very fine biliary ducts, which I 

 have, as stated above, observed microscopically, with a diameter of 0-01 

 and a cavity of 0'0033 of a line, are identical with a part, at all events, 



from that of Man. In the lowest forms of animal life, where no distinct digestive apparatus 

 exists no trace, according to Prof. Leidy, of biliary structure is observable. When we rise 

 a little in the scale, as in the Polypi, certain cells forming part of the digestive cavity, pos- 

 sess the power of secreting a fluid analogous to bile. In many of the Annelida, we find 

 appended to the sides of the alimentary canals caeca lined by small cells, which probably 

 secrete a biliary fluid. A similar substance is formed in the Myriapoda by the cells of the 

 long and delicate tubes, which empty into the intestine. 



In insects the liver is formed of distinct, filiform, tortuous tubes, opening near the pyloric 

 extremity of the stomach by separate orifices into the sides of the alimentary canal. These 

 tubes consist of a transparent, amorphous, basement membrane, the inner surface of which 

 is covered by secreting cells. These cells are generally round or oval, and contain molecular 

 matter, numerous fine oil-globules, a central granular nucleus and a transparent nucleolus. 

 The tubes are filled with fine granules and a great amount of oil 5 extremely minute respi- 

 ratory tracheae are distributed to them. 



In the Crustacea, as in the Cray-fish, the liver is formed of two large lobes, one on each 

 side of the intestine, united by an isthmus. Each lobe consists of conical caeca, composed 

 of a sac of basement membrane lined with numerous secreting cells, of a more or less 

 polygonal form. These cells have an average diameter of O2 of a line ; they increase in 

 size from the bottom of the caecum upwards, and obtain a gradual addition of oil-globules. 

 The central cavity of the caeca is filled with fat-globules and a finely granular mass. 



In the Molluscs, as in the Snail and Slug, the liver is formed of several lobes, subdivided 

 into lobule?, which are composed of numerous bulbiform caeca of a polygonal shape. These 

 caeca do not differ in structure from the caeca in the liver of the Crustacea. They are lined 

 with cells of an average diameter of 0'2 to 0'4 of a line, filled with oil-globules, and contain- 

 ing a granular nucleus with a hard transparent nucleolus. Around the caeca there is a net- 

 work of bloodvessels, to the parietes of which, Prof. Leidy states, minute oil-globules and 

 nucleated fat-cells are frequently attached. 



In the Vertebrata, the facts observed by Prof. Leidy confirm the researches of Kiernan and 

 other recent observers. He considers, however, the biliary tubes to commence within the 

 lobules, arid not merely, as stated by Koiliker, as a layer of cells, but as a network of distinct 

 tubules lined with a basement membrane and an epithelium. Their diameter in different 

 animals is generally two and a half times the size of the secreting cells. (Vid. Leidy, 

 " Researches into the Comparative Structure of the Liver," in American Journal of Medical 

 Sciences, January 1848.) DaC.] 



