

m 



COMPARATIVE STRUCTURE OF THE LIVER. 401 



li \j 



s 



fices. I n Insects and other air-breathing Articulata, however, the 



;ver is much less developed; and its type remains much simpler, 

 e usually find it consisting of a small number of caecal tubuli, which 

 open separately into the intestinal canal, just below the stomach. These 

 tubuli are often so long, as to pass several times from one extremity of 

 the visceral cavity to the other, being doubled upon themselves ; in 

 other instances, we find that the principal tube or canal is beset with 

 rows of short follicles, somewhat in the manner of Fig. 111. But they 

 never cluster together so as to form a solid glandular mass. The low 

 development of the liver, in these animals, bears an evident relation 

 with the high development of their respiratory apparatus ; whilst, the 

 respiration being comparatively feeble in the aquatic Mollusca and 

 Crustacea, the development of the liver in those classes is enormous. 



7*21. There is much difficulty in ascertaining the mode in which the 

 elementary constituents of the Liver are arranged, in the fully-developed 

 condition of that organ in the higher Yertebrata. At an early period of 

 its development, as already remarked, it may be easily shown to con- 

 sist, in the Fowl, of a series of distinct caeca, clustered round a projec- 

 tion from the intestinal canal, and opening separately into it (Fig. 108) ; 

 and it is a peculiarly interesting fact, that this very condition should 

 exist as the permanent form of the Liver, in that curious little fish, the 

 AmpMoxus or Lancelot, which retains the embryonic type in so many 

 parts of its conformation. In the Tadpole, again, the distinct caeca are 

 very evident (Fig. 118) ; but here we see that the projection of the 

 intestinal canal, instead of being a simple wide caecum, has become 

 extended in length and contracted in diameter, at the same time di- 

 viding and subdividing, so as to form an arborescent excretory duct, 

 whose ramifications extend through the entire glandular mass. In this 

 manner, then, is formed the complex system of hepatic ducts, which we 

 find in the liver of the higher Vertebrata, branching out from the main 

 trunk. But the mode in which the ultimate ramifications of these are 

 arranged, and their relations with the secreting cells which make up the 

 parenchyma of the gland, have not yet been fully elucidated. The fol- 

 lowing are the principal facts, that have been ascertained on the sub- 

 ject. 



722. The entire Liver is made up of a vast number of minute lobules, 

 of irregular form, but about the average size of a millet-seed. Each of 

 these lobules contains the component elements 7 of which the entire 

 organ is made up ; namely, branches of the hepatic artery and vein, 

 branches of the portal vein, branches of the hepatic ducts, and secreting 

 cells. The lobules are connected together in part by areolar tissue, but 

 in great part by the anastomosis of the blood-vessels and hepatic ducts, 

 which supply the adjoining lobules ; indeed there is frequently no 

 definite division of the glandular substance into lobules, other than that 

 which is marked out by the arrangement of these canals (Figs. 119 and 

 121). The branches of the Hepatic Artery are principally distributed 

 upon the walls of the hepatic ducts, and upon the trunks and branches 

 of the portal and hepatic veins, supplying them with their vasa vasorum; 

 also upon Glisson's capsule and its prolongations into the substance of 

 the liver, which prolongations form the greatest part of the connecting 



26 



