461 LIVER. [CHAP, xxxni. 



side of it, i.e., where the blood is leaving it, so that the blood circu- 

 lates in the reverse direction to that in which the bile must flow. 



The Liver in Invcrtebrata. The liver is one of the most constant glandular 

 organs, being met with, in some form, in all animals provided with a digestive 

 cavity. In the polyps, the liver is represented by some coloured cells round 

 the stomach. In many of the annelids, clusters of biliary cells are seen sur- 

 rounding the csDcal prolongations of the digestive cavity. In the Eolis (one 

 of the nudibranchiate gasteropodous mollusks) a somewhat similar arrange- 

 ment is observed, the follicles of the alimentary tube being prolonged into 

 the papilla?, covering the dorsal surface of the animal. In most other mollusks, 

 however, the liver exists as a distinct organ, and is composed of branched 

 follicles arranged round terminal ducts. The follicles contain coloured cells, 

 in which oil globules are often present in considerable number. In many of 

 the Crustacea, the liver is detached from the intestinal walls, and consists of 

 numerous large ccoca (Fig. 215), which pour their contents into small ducts 

 although in others it seems to consist simply "of cells arranged in follicles 

 which are connected with the intestine, as in the lowest classes. In insects, 

 the hepatic organ takes the form of simple or branched tubes, from two to 

 six in number, which open into the intestine. According to our observations, 

 the cells do not appear to be arranged round the tube, so as to leave a distinct 

 central channel, as in the uriniferous tubule, but lie within the basement 

 membrane, without order or regularity, often completely filling the tube, and 

 not unfrequently, from their largo size, causing it to bulge. We shall pre- 

 sently see that a very similar arrangement exists in the tubular network of 

 basement membrane, which contains the liver-cells in vertebrate animals. 



Throughout the whole animal series, the liver consists essentially of cells 

 containing colouring matter, and usually oil globules, which lie within a tube, 

 or follicle of basement membrane, continuous with the alimentary canal. 



Having premised these general points, we shall now proceed to 

 consider the anatomy of the liver more in detail. 



Lobules of the Liver. The terminal twigs of the portal veins 

 and the commencing radicles of the hepatic vein, thus distributed 

 through the liver with a definite thickness of capillary plexus with 



nucleated bile-cells interposed, are 

 further arranged in such a manner as 

 t ( that the intervening mass is gathered, 



not into a folded sheet, but into a 

 great number of small portions, termed 



f ';> lobules. These lobules are apparent to 



/"!;" the eye in many animals; but in the 



pig they are each of them invested by a 



X -"^M JlV \ ^^ ' --- 



separate and distinct membranous en- 



Portion of fibrous capsule of a lobule of Velope Or Capsule, which IS Composed of 

 the pie's liver, showing arrangement of n i n -> ,. /TV ^ ON T , i 



the fibres- 215 diameters. delicate fibrous tissue (Fig. 218) . In this 



animal, each lobule may be regarded as 



