4.50 



SECRETION. 



and biliary organs are so intimately connected 

 in this genus, that it is not easy to point out 



Fig. 316. 



Biliary apparatus of Eolis. 



L pa 

 gland b and 



A, branchial papilla of E. papillosa, exhibiting the 

 the duct c ; also an ovate vesicle, a, 



organ of defence, and at d the wall 

 the inner sheath ; B, branchial papilla of E. 

 Farrani, showing the same parts. (After Alder 

 and Hancock.) 



the limits of each ; they appear to differ in 

 different species. In E. papillosa, the central 

 canal is evidently a continuation of the sto- 

 mach, and the plicated internal membrane is 

 not only continuous throughout it, but also 

 passes into the lateral branches, which thus 

 appear to form part of the same organ. On 

 the other hand, we find in some species 

 coloured granules, similar to those of the 

 papillae, partially lining the ramifications, as 

 in E.gracilis and others ; while in E. despecta, 

 the central canal, all the ramifications, and the 

 glands of the papillae, are coloured and gra-. 

 nulated alike, implying a greater diffusion of 

 the biliary function. The food, after being 

 partially digested in the stomachal pouch, is 

 driven in detached portions through the 

 alimentary system, by the alternate contrac- 

 tions of the pouch and great trunks leading 

 from it ; these contractions are only of a 

 nature to produce an oscillatory motion, 

 which serves to promote that intimate mixture 

 of the alimentary matters with the hepatic 

 and other secretions, necessary to the process 

 of digestion." * 



The intimate structure of the liver of Ver- 



* Alder and Hancock's Nudibranchiate Mollusca, 

 Part in. 



tebrated animals is much more difficult of 

 elucidation, and can scarcely be said to be yet 

 satisfactorily determined. The organ presents 

 more and more, as we ascend the series, a 

 solid parenchymatous texture, which strikingly 

 contrasts with its loosely tabulated racemose 

 aspect, even in the highest Invertebrata. 

 There is not the least difficulty in demon- 

 strating that this parenchyma is composed of 

 cells, which correspond in the nature of their 

 contents, and, therefore, in their functional 

 character, with those contained within the 

 hepatic follicles of the Invertebrata ; but the 

 point of obscurity is the relation of these cells 

 to the biliary ducts, the arrangement of whose 

 ultimate ramifications has been rather a matter 

 of surmise and inference, than of actual obser- 

 vation. It is very interesting to find, however, 

 that in the lowest known Vertebrate the liver 

 exists under the same rudimental and diffused 

 type, as that which it exhibits in the lower Arti- 

 culata. In the Amphioxus, or lancelet, the only 

 vestige of a distinct hepatic organ is a large 

 caecum prolonged from the stomach, which is 

 lined with greenish-yellow cells. But it is 

 pointed out by Miiller, that the intestinal 

 canal itself has a layer of similar cells in its 

 walls, so that the organ would seem to have 

 the same diffused condition as that which it 

 presents in the earth-worm. In all other 

 fishes, however, the liver is a well-defined 

 conglomerate gland, even the Myxinoids 

 presenting a liver nearly as fully developed as 

 that of the higher fishes, so that there is not 

 here any such complete gradation as we usu- 

 ally meet elsewhere. Dr. T. Williams states 

 that he has succeeded in tracing the ducts to 

 their ultimate terminations in the liver of the 

 Sole (Solea vulgaris), and the Flounder (Pla- 

 tessaflexus) ; and he describes them as rami- 

 fying like those of the Mollusca, and as ending 

 in tubular caeca, without vesicular expansions. 

 Within these caeca are found the hepatic 

 cells, which usually, as in the Invertebrata, 

 contain a large quantity of fat.* There is a 

 remarkable diminution in the proportion of 

 the adipose contents of the hepatic cells, and 

 an increase in the granular constituents, in 

 the class of Reptiles ; and in Birds there is 

 an almost total absence of adipose particles. 

 The ultimate distribution of the bile-ducts, and 

 their relation to the parenchyma, seem to be 

 the same as in the Mammalia. 



In the Mammalia, the liver is more or less 

 distinctly divisible into minute lobules, each 

 of them composed of a parenchyma of hepatic 

 cells, through which the blood-vessels are dis- 

 tributed in a close and solid plexus. The 

 hepatic cells appear to occupy the entire space 

 left in the meshes of this plexus, the bile- 

 ducts having been usually regarded as not 

 traceable, under any form, into the interior of 

 the lobule. Mr. Kiernan, however, has always 

 regarded the bile-ducts as forming a plexus in 

 the substance of the lobule, interlacing with 

 the plexus of capillaries; his belief being 

 chiefly founded on the anastomotic distribu- 



* Guy's Hospital Keports, 1846, p. 323. 



