THE LIVER. SECRETION OF BILE. 



581 



2. The Liver. Secretion of Bile. 



623. The Liver is probably more constantly present, under some form or 

 other, throughout the entire animal series, than any other gland. Its form and 

 condition vary so greatly, however, in different tribes, that, without a knowledge 

 of its essential structure, we should be disposed to question whether any identity 

 of character exists among the several organs which are regarded as Hepatic. It 

 is, in fact, the presence of bile-secreting cells, that must be held to constitute a 

 Liver ; and these may be scattered over the general lining membrane of the ali- 

 mentary canal, or may be restricted within follicles which are formed by depres- 

 sions of it ', these follicles, again, may be multiplied in some particular spot, so as 

 to be aggregated into a mass, or may be extended into long tubes. In all the In- 

 vertebrata, however, the Liver is obviously conformable to the general type of 

 glandular structures ; the hepatic cells being in immediate relation with a base- 

 ment-membrane, and being discharged upon a free surface. This will be readily 



Fig. 149. 



Lobule of Liver of Squilla Mantis: A, exterior ; B, the same cut open. 



understood from an examination of any one of the higher forms of it, such as that 

 presented in the liver of the Crab (Fig. 149), which, like the liver of the Mollusca 

 generally, is a lobulated glandular mass, formed by the aggregation of a multi- 

 tude of follicles with distinct caecal terminations ; these follicles discharging their 

 secreted products into cavities which occupy the centre of the lobules, whence 

 they are collected by the ducts which discharge them into the alimentary canal. 

 On a careful examination of these follicles (Fig. 150), and a comparison of 

 the size and contents of the cells at the bottom and towards the outlet, it becomes 

 evident that the cells originate in the former situation, and gradually increase 

 in size as they advance towards the latter. It is also to be observed that the 

 cells which lie deepest in the caecum (a, 6) contain for the most part the yellow 

 granular matter, which may be regarded as the proper biliary secretion ; but 

 as they increase in size, there is also an increase in the quantity of oil-globules 

 which they contain (c), until past the middle of the follicle, where they are 

 found full of oil, so as to have the appearance of ordinary fat-cells (d, e). From 

 this it happens, that when an entire caecum is examined microscopically, its lower 

 half appears filled with a finely granular matter, intermingled with nucleated 

 particles ; and the upper half with a mass of fat-cells, whose nuclei are ob- 

 scured by the oily particles. 1 In Vertebrated animals, however, the Liver seems 



1 See Dr. l^eidy's " Researches into the Comparative Structure of the Liver," in 

 Journ. of Med. ScL," Jan. 1848. 



Amer. 



