746 LIVER OF FROG. [BOOK n. 



made up of a number of tubules, which repeatedly not only 

 branch but also anastomose (Fig. 91), and among which run the 

 capillary blood vessels uniting the branches of the portal with those 

 of the hepatic vein. There is no very obvious division into 

 lobules ; indeed in a section of small area the tubules appear to 

 run irregularly; nevertheless they have a definite arrangement 

 around the branches of the hepatic vein. Both longitudinal and 

 transverse sections of one of these tubules shew that it is lined 

 with large wedge-shaped cells, leaving a very narrow, almost linear 

 but still distinct lumen. Around the tubule is disposed a network 

 of capillaries, and, as in the alveolus of an ordinary gland, the 

 blood vessel is separated from the lumen of the tubule by the 

 thickness of an entire cell. Each cell possesses a rounded nucleus 

 which lies in the outer part of the cell nearer to the blood vessel 

 than to the lumen; and we may mention here, though we shall 

 return to the point later on, that the cell-substance contains a 

 number of granules which are sometimes scattered throughout 

 the cell and sometimes aggregated near the lumen. The hepatic 

 cell of the frog repeats in fact the main characters of the secreting 

 cell of an ordinary gland, of a pancreatic cell for example. The 

 tubules moreover when traced are found to end in ducts, the 

 (secreting) hepatic cells suddenly changing to cubical and then 

 to columnar (conducting) cells, which in the larger ducts bear cilia. 

 In other words, the hepatic tubules of the frog are alveoli, differing 

 from the alveoli of an ordinary gland, in that they repeatedly 

 anastomose as well as branch, and in that the lumen is very 

 narrow and, since it also branches and anastomoses, forms a 

 network of fine passages. 



From a liver such as that of the frog the change to the arrange- 

 ment of the mammalian liver is one of degree only. The branching 

 and anastomosing of the tubules is still more frequent and complete 

 and the lumina of the tubules still narrower, so much so that each 

 cell, as it were, takes part in several tubules, and the network of 

 the lumina or bile-canaliculi is so close set that the meshes are of 

 about the same width as the hepatic cells. The blood vessels 

 moreover are more abundant, and by the establishment of an 

 arrangement whereby interlobular (portal) veinlets send capillaries 

 to converge radially to an intralobular (hepatic) veinlet, the hepatic 

 substance, instead of as in the frog being distributed more or less 

 uniformly, is divided into a number of small areas, the hepatic 

 lobules. 



452. Concerning the nerves of the liver we shall say what 

 there is to be said when we come to consider the action of the 

 nervous system on the hepatic metabolic processes. 



With lymphatics the liver is well provided. Within the lobule 

 lymph-spaces exist between the walls of the vascular network and 

 the outer margins of the hepatic cells, and at the circumference 

 of the lobule these spaces open into definite lymphatic vessels 



